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Carbon Dioxide Assessment Committee 



William A. Nierenberg, Scripps Institution of Oceanography , Chairman 
Peter G. Brewer, Woods Hole Oceanographic Institution (on leave with 

the National Science Foundation) 
Lester Machta, Air Resources Laboratory, National Oceanic and 

Atmospheric Administration 
William D. Nordhaus, Yale University 
Roger R. Revelle, University of California, San Diego 
Thomas C. Schelling f Harvard University 
Joseph Smagorinsky, Princeton University 

Paul E. Waggoner, Connecticut Agricultural Experiment Station 
George M. Woodwell, Marine Biological Laboratory, Woods Hole, 

Massachusetts 



Consultants 

David A. Katcher, Chevy Chase, Maryland 
Gary W. Yohe, Wesleyan University 



Staff 

John S, Perry, Executive Secretary 
Jesse H. Ausubel, Staff Officer 
James A. Tavares 
Adele King Malone (to 5/82) 



iii 



UNIVERSITY LIBRARIES 

CARNEGIE-MELLON UNIVERSITY 

PITTSBURGH, PENNSY"" 1 /::.' ]'-2l3 



Detection and Monitoring Working Group 



Gunter Weller, University of Alaska, Chairman 

D. James Baker, Jr., University of Washington 

W. Lawrence Gates, Oregon State University 

Michael C. MacCracken, Lawrence Livermore Laboratory, 

Syukuro Manabe, Geophysical Fluid Dynamics Laboratory, National Oceanic 

and Atmospheric Administration 
Thomas H. Vender Haar, Colorado State University 

John S. Perry, Executive Secretary 



iv 



Climate Research Committee 



Joseph Smagorinsky, Princeton University, Chairman 

Tim P. Barnett, Scripps Institution of Oceanography 

Harry L. Bryden, Woods Hole Oceanographic Institution 

Isaac Held, Geophysical Fluid Dynamics Laboratory, National Oceanic and 

Atmospheric Administration 

Frederick T. Mackenzie, Northwestern University 
James C. McWilliams, National Center for Atmospheric Research 
Norman Phillips, National Oceanic and Atmospheric Administration 
Veerabhadran Ramanathan, National Center for Atmospheric Research 
Jagadish Shukla, Goddard Space Flight Center, National Aeronautics and 

Space Administration 

Thomas H. Vonder Haar, Colorado State University 
John M. Wallace, University of Washington 
Ferris Webster, University of Delaware 
Gunter E. Weller, University of Alaska 



Board on Atmospheric Sciences and Climate 



Thomas F. Malone, Butler university. Chairman 

Ferdinand Baer, university of Maryland 

Louis J. Bat tan, university of Arizona 

Werner A, Baum, Florida State University 

Robert A. Duce, university of Rhode Island 

John A. Eddy, National Center for Atmospheric Research 

Peter V. Hobbs, University of Washington 

Francis S. Johnson, University of Texas, Dallas 

Robert W. Kates, Clark University 

Michael B. McElroy, Harvard University 

James C. McWilliams, National Center for Atmospheric Research 

Volker A. Mohnen, State University of New York, Albany 

Andrew F. Nagy, University of Michigan 

William A. Nierenberg, Scripps Institution of Oceanography 

Roger R. Revelle, University of California, San Diego 

Juan G. Roederer, University of Alaska 

Norman J. Rosenberg, University of Nebraska 

Stephen H. Schneider, National Center for Atmospheric Research 

Joseph Smagorinsky, Princeton University 

John w. Townsend, Fairchild Space & Electronics Company 

Thomas H. Vender Haar, Colorado State University 



VI 



Commission on Physical Sciences, Mathematics, and Resources 



Herbert Friedman, National Research Council, Chairman 

Elkan R. Blout, Harvard Medical School 

William Browder, Princeton University 

Bernard F. Burke, Massachusetts Institute of Technology 

Herman Chernoff , Massachusetts Institute of Technology 

Mildred S. Dresselhaus, Massachusetts Institute of Technology 

Walter R. Eckelmann, Sohio Petroleum Company, Dallas, Texas 

Joseph L. Fisher, Secretary of Human Resources, Office of the Governor, 

Richmond, Virginia 

James C. Fletcher, Burroughs Corporation, McLean, Virginia 
William A. Fowler, California Institute of Technology 
Gerhart Friedlander, Brookhaven National Laboratory 
Edward A. Frieman, Science Applications, Inc., La Jolla, California 
Edward D. Goldberg, Scripps Institution of Oceanography 
Charles L. Hosier, Jr., Pennsylvania State University 
Konrad B. Krauskopf, Stanford University 
Charles J. Mankin, Oklahoma Geological Survey 
Walter H. Munk, University of California, San Diego 
George E. Pake, Xerox Research Center, Palo Alto, California 
Robert E. Sievers, University of Colorado, Boulder 
Howard E. Simmons, Jr., E.I. du Pont de Nemours & Co., Inc. 
John D. Spengler, Harvard School of Public Health 
Hatten S. Yoder, Carnegie Institution of Washington 

Raphael G. Rasper, Executive Director 



vii 



Foreword 



The Energy Security Act of 1980, while focused on the development of 
synthetic fuels, also called for examination of some of the environ- 
mental consequences of their development. One such consequence 
perceived by the Congress was the buildup of carbon dioxide (CO 2 ) in 
the atmosphere, and the National Academy of Sciences (NAS) and the 
Office of Science and Technology Policy (OSTP) of the Executive Office 
of the President were requested to prepare an assessment of its 
implications. 

Concern about the atmosphere's carbon dioxide and its influence on 
climate dates back to the last century. In the 1970s, however, with 
recognition of a growing world population and increasing per capita use 
of energy, attention markedly heightened. In 1977 the National Research 
Council issued a report. Energy and Climate, 1 prepared by a panel 
chaired by Roger Revelle, calling for an intensified program of research 
on C02 At around this time, the federal government began expanding 
its concern with C0 2 , primarily through a research and assessment 
program in the Department of Energy. In a congressional symposium on 
CO 2 and energy policy in 1979 some scientists expressed the fear that 
atmospheric C0 2 could double by the first decade of the twenty-first 
century if coal and fossil-based synthetic fuels were vigorously 
exploited. 

Such concerns and the increasing volume of research results led the 
Congress and the Executive to ask the NAS to consider anew various 
aspects of the issue. In July* 1979, a brief preliminary statement 
about CO 2 and energy policy was released by the Academy, and later in 
that same summer a Panel of the Climate Research Board chaired by the 
late Jule Charney undertook an evaluation of the models being used to 
estimate likely effects of C0 2 on climate. 2 In the following 
winter and spring, a committee chaired by Thomas C. Schelling and 
including several other members of the current Committee considered 



1 Geophysics Study Committee (1977) . Energy and Climate. National 
Academy Press, Washington, D.C. 

2 National Research Council (1979) . Carbon Dioxide and Climate; A 
Scientific Assessment. National Academy Press, Washington, D.C. 

ix 



some of the economic and social aspects of increase in C0 2 . 3 At 
the same time, in April 1980, the Senate Committee on Energy and 
Natural Resources convened a hearing on the issue. 

In this climate of concern, the Energy Security Act of 1980 was 
passed, calling upon OSTP to request a study by the Academy that would 
deal with, among others, the following issues: 

A comprehensive assessment of C0 2 release and impacts of 
CO 2 increase; 

Development of an international research and assessment program 
and definition of the U.S. role; 

Analysis of domestic resource requirements for international 
and domestic programs; 

Evaluation of the U.S. government C0 2 program; and 

Assessment of the need for periodic reports and a long-term 
assessment program. 

Annex 3 of this report contains the relevant subtitle of the 
legislation. 

As congressional interest was mounting, the Climate Research Board 
requested one of its members, William A. Nierenberg, to monitor 
developments and to advise the Board on appropriate actions. In 
response to the congressional mandate, the Carbon Dioxide Assessment 
Committee (CDAC) was formed under his leadership to develop a plan to 
accomplish the requested study. 

With support from OSTP, the Committee developed a preliminary plan, 
which was provided to OSTP for comment in January 1981. The change of 
administration at that time slowed the discussions of the study's scope 
and objectives. However, in the summer of 1981 a plan of action was 
agreed on by which the HAS and OSTP could respond to the congressional 
request for an independent and comprehensive assessment. The report 
would comprise two major parts: (1) an overview or synthesis repre- 
senting the views of the Committee as a whole on the issue and (2) a 
group of papers each addressing a specific topic or problem area and 
prepared by an individual committee member or a specialist group. 

The Committee began its work in September 1981. Over the next 2 
years, the Committee members met four times (September 28-29, 1981, 
Washington, D.C.; March 25-26, 1982, La Jolla, California; September 
20-21, Berkeley Springs, West Virginia; January 13-14, 1983, Washington, 
D.C.) to monitor progress on their specific topics and to develop their 
collective views. 

A number of considerations went into the design of the Committee and 
the selection of additional experts to contribute to its work. Com- 
petence was sought in each of the major subject areas of the question, 



9 Letter report of the Ad Hoc Study Panel on Economic and Social 
Aspects of Carbon Dioxide Increase, T. C. Schelling, Chairman. April 
18, 1980. Climate Research Board, National Academy of Sciences, 
Washington, D.C. 



as well as experience with assessment of long-range issues. A balance 
of viewpoints about environmental issues was sought. Finally, con- 
tinuity was maintained with previous, related NRC efforts (Energy and 
Climate, Energy in Transition,* the Schelling report) . Additional 
experience and skills were provided by consultants to the Committee and 
by ad hoc workshops and groups convened with the assistance of other 
NRC units. 

Before the reader turns to the contents of this report/ the question 
might be raised: Why another C02 report, apart from the legislative 
request? The C0 2 issue has been probingly addressed by many indi- 
viduals and groups in recent years. One could mention, for example, 
Carbon Dioxide Review; 1982, 5 Carbon Dioxide from Coal Utilization, 6 
On the Assessment of the Role of CO-? on Climate Variations and Their 
Impact, 7 The CO 2 -Climate Connection, 8 The Long -Term Impacts of 
Increasing Atmospheric Carbon Dioxide Levels, 9 Interactions of Energy 
and Climate, 10 as well as the NEC's own Energy and Climate, Charney 
report, and Schelling report. There are at least two reasons to con- 
tribute to the growing volume of literature on the CC>2 issue. One is 
that research developments are occurring at a rapid rate, as we see in 
many sections of the report that follows. A second reason is that the 
focus and perspectives of the reports on the CO2 issue differ. A 
major distinguishing feature of Changing Climate is that it represents 
a sustained attempt by a group with a wide range of expertise to achieve 
a comprehensive and internally consistent assessment. 

On behalf of the Board and the Academy, I wish to express our appre- 
ciation to Professor Nierenberg, the members of the Committee, and the 
many other participants in this study for their individual and collec- 
tive contributions to this report. As described in the Historical Note 
(Annex 2) , the carbon dioxide issue has been with us for a long time, 



"National Research Council (1979). Energy in Transition 
1985-2010. Final Report of the Committee on Nuclear and Alternative 
Energy Systems (CONAES) . W. H. Freeman, San Francisco. 

5 W. C. Clark, ed. (1982). Carbon Dioxide Review; 1982. Oxford 
U. Press, New York, 469 pp. 

6 1. M. Smith (1982). Carbon Dioxide from Coal Utilization. 
Technical Information Service, International Energy Agency, Paris. 

7 WMO/ICSU/UNEP (1981). On the assessment of the role of CO 2 on 
climate variations and their impact. (Based on meeting of experts, 
Villach, Austria, November 1980.) World Meteorological Organization, 
Geneva, January 1981. 

8 G. B. Tucker (1981) . The COo-Climate Connection. Australian 
Academy of Science, Canberra. 

9 G. J. MacDonald, ed. (1982). The Long-Term Impacts of Increasing 
Atmospheric Carbon Dioxide Levels. Ballinger, Cambridge, Mass., 263 pp. 

10 W. Bach, J. Pankrath, and J. Williams, eds. (1980). 
Interactions of Energy and Climate. Reidel, Dordrecht, The Netherlands. 

xi 



and it will undoubtedly maintain a prominent place on our agenda for a 
long time to come. We continue to need well-coordinated programs of 
research, productively interdisciplinary in character and broadly 
international in scope. As Professor Nierenberg indicates in his 
Preface, this report is best viewed as one stepping-stone on a long 
pathway into the future. We are confident, however, that it will prove 
to be a solid step on that pathway toward more complete understanding 
of this complex issue. 

Thomas F. Malone, Chairman 

Board on Atmospheric Sciences and Climate 



xii 



Preface 



There is a broad class of problems that have no "solution" in the sense 
of an agreed course of action that would be expected to make the problem 
go away. These problems can also be so important that they should not 
be avoided or ignored until the fog lifts. We simply must learn to deal 
more effectively with their twists and turns as they unfold. We require 
sensible regular progress to anticipate what these developments might 
be with a balanced diversity of approaches. The payoff is that we will 
have had the chance to consider alternative courses of action with some 
degree of calm before we may be forced to choose among them in urgency 
or have them forced on us when other perhaps better options have been 
lost. Increasing atmospheric C0 2 and its climatic consequences 
constitute such a problem. 

Research developments are taking place rapidly in this area. In the 
pages that follow we report our understanding of the status of a number 
of selected, critical aspects and comment on how well we think the 
overall attack on this complex matter is proceeding. Our stance is 
conservative: we believe there is reason for caution, not panic. Since 
understanding and proof of what is happening to climate as a result of 
practices that load the atmosphere with C0 2 may come too late to allow 
for corrective action, we may not be able to wait to make certain there 
is a best course. Thus, we must proceed in a manner that keeps open 
our major options on energy development and use, on water management, 
agricultural adjustment, and other relevant activities, as we move from 
one set of uncertainties to another. We make an effort in this report 
to point the way as we see it today. 

A range of approaches was employed in developing the Committee's 
report. For example, in the study of possible future CO 2 emissions, 
a review of earlier research was commissioned and a new model was 
constructed to remedy some of the shortcomings of previous work. The 
carbon cycle was addressed through individual reviews of its oceanic, 
atmospheric, and biotic components, together with model-based sensitiv- 
ity analyses. In the area of agriculture a survey was undertaken, and 
several outside experts were convened in a small informal workshop to 
address the relationship of climatic change to crop yield. A group was 
also convened, in Athens, Georgia, in May 1982, in conjunction with a 
meeting organized by the American Association for the Advancement of 

xiii 



Science on direct effects of C0 2 on plants. Small, informal work- 
shops were also convened in the areas of hydrology and land surface 
processes (La Jolla, March 16-17, 1982) , Antarctic Ice (La Jolla, March 
18-19, 1982) , and Arctic Ice (Philadelphia, June 1-2, 1982) . Questions 
of sea- level rise were explored with assistance from experts from the 
Samenwerkende Instellingen ten Behoeve van beleidsanalytische Studies 
(SIBAS) , Delft, The Netherlands. The general area of scenario construc- 
tion and evaluation benefited from the participation of three Carbon 
Dioxide Assessment Committee (CDAC) members in a workshop on this topic 
at the International Institute for Applied Systems Analysis in 
Laxenburg, Austria, in July 1982. 

The Committee recognized early that the central issue of C0 2 
effects on climate would require intensive review. Thus, the Board's 
Climate Research Committee was asked to re-examine and update the work 
of the 1979 Charney panel. A panel chaired by Joseph Smagorinsky, a 
member of the CDAC and Chairman of the Climate Research Committee (CRC) , 
undertook this task and issued its report. Carbon Dioxide and Climate; 
A Second Assessment, in 1982. This document should be considered an 
integral part of the present study; its conclusions are reproduced in 
this volume together with brief supplementary comments. The CRC was 
also asked to consider the detection of climatic change induced by 
C0 2 and the monitoring of climatic variables. With the advice of the 
CRC, a group of experts, including a number of CRC members, was asked 
to contribute to the report. These experts met several times during 
1982 under the leadership of Gunter Weller, and the results of their 
conferring form Chapter 5 of this volume. 

The scope of the Committee's report is broad, and I believe it meets 
as fully as a small group could the congressional request for a "compre- 
hensive" assessment of the C0 2 issue. A truly complete assessment of 
the C0 2 issue might involve most or all of a very wide range of 
elements, including identification of various risks and prospective 
changes; estimation of probabilities of their occurrence; linkage of 
such events with various environmental and social consequences; and 
evaluation of the risks by comparison with costs, with other risks, 
with benefits, with alternative ways of reducing risks, or with risks 
of substitute activities. In this report the Committee does attempt to 
shed a little light on all of these aspects of the C0 2 issue: C0 2 
emissions and concentrations are projected? possible climatic changes 
are assessed; implications of increasing C0 2 and climatic change for 
agriculture and water, sea level, and other selected areas are examined, 
including possible impacts that might have a low probability but a high 
cost; and possible consequences are evaluated against historical 
experiences and other current and future problems. 

However, the report certainly does not exhaust the issue. A number 
of possibly important problems are left for future investigators. 
Among such problems are the effects of non-C0 2 greenhouse gases on 
climate, effects of altered climate on agriculture and water outside 
the United States, and the feasibility of alternative nonfossil energy 
strategies. Furthermore, as interest in synfuels development dimin- 
ished, the CDAC chose to place less emphasis on this aspect of the 



xiv 



CO 2 issue than would have been the case if the report had been 
prepared largely during 1979 and 1980. While the report offers 
estimates of probabilities in several areas, it is not always possible 
to do so in a meaningful way. Consistent treatment of uncertainty in 
each of the aspects of this heterogeneous issue remains elusive, but I 
believe that this report makes substantial progress in this regard. 

The CO 2 issue is so diverse in its intellectual components that no 
individual may be considered an expert on the entire problem. For this 
reason, as noted above, the CDAC prepared or commissioned separately 
authored and separately peer-reviewed papers in each area, with no 
attempt to force unanimity of style or of views. For the same reason, 
the Committee members felt themselves incapable of judging and endors- 
ing as a group the details of each paper's analysis and findings. Thus, 
each paper should be viewed primarily as the product of its individual 
author or authors, having had the review and comment of the Committee 
members and other reviewers but not enjoying the unanimity of conclu- 
sions possible in a more homogeneous and less controversial topic. 
However, the Committee's work did reveal a large core of views, find- 
ings, conclusions, and recommendations on a more general level, which 
all members could wholeheartedly and responsibly endorse. These are 
presented in the Synthesis of the report. Despite the existence of 
some areas of continuing controversy, such as the carbon cycle, there 
are no major dissents with respect to the contents of this assessment. 

There continues to be an outpouring of fact, interpretation, con- 
jecture, and proposals for research and policy having to do with the 
effect of increasing carbon dioxide and its consequences. Periodically, 
it will be well worth taking stock. Where are we? What is most impor- 
tant or troublesome? What should we be doing now, although treatment 
of the issue is full of uncertainties, to avoid the haste and waste of 
being forced to shoulder a burden later, when our best choices may have 
been foreclosed? What opportunities does the prospect of C0 2 -induced 
changes offer societies? Are we doing anything now that will look like 
a mistake later on? This report is the result of one effort to gain 
insights into these questions. We hope that others in the United States 
and other countries will continue to ask them. Such sustained question- 
ing may be the best insurance we can buy. 

The support that the CDAC has received from the Congress and the 
federal government throughout its work has been gratifying. The Office 
of Science and Technology Policy (OSTP) of the Executive Office of the 
President, the Department of Energy (DOE) , and the National Science 
Foundation (NSF) provided the necessary means, as well as ready coopera- 
tion on access to information and expertise. Richard Meserve and 
Thomas Pestorius of OSTP, David Slade and Frederick Koomanoff of DOE, 
and Bernard Stein of NSF deserve special mention. 

I am sure that all the members of the Committee also share my 
gratitude to the many experts who let us draw upon their knowledge and 
wisdom through many ad hoc means. Our sister Climate Research 
Committee deserves many thanks for organizing groups to address the 
questions of climate modeling, detection of climate changes, and 



xv 



monitoring. Some 30 volunteer reviewers provided trenchant and most 
helpful comments that greatly strengthened the report. Finally, we 
appreciate greatly the support provided by the NRC staff: particularly 
Jesse H. Ausubel and John S. Perry of the Board on Atmospheric Sciences 
and Climate (BASC) ; and also Robert S. Chen of BASC's predecessor, the 
Climate Board; James A. Tavares of the Board on Agriculture; and Gary 
Yohe and David Katcher, consultants to BASC. 

William A. Nierenberg, Chairman 
Carbon Dioxide Assessment Committee 



xvi 



Contents 



EXECUTIVE SUMMARY 



SYNTHESIS 

Carbon Dioxide Assessment Committee 

1.1 INTRODUCTION, 5 

1.2 THE OUTLOOK, 9 

1.2.1 Future C0 2 Emissions, 9 

1.2.2 Future Atmospheric C0 2 Concentrations, 14 

1.2.3 Changing Climate with Changing C0 2 , 27 

1.2.4 Detection of CO2-Induced Changes, 32 

1.2.5 Agricultural Impacts, 36 

1.2.6 Water Supplies, 40 

1.2.7 Sea Level, Ar^fcarctic, and Arctic, 40 

1.3 SERIOUSNESS OF PROJECTED CHANGES, 44 

1.3.1 Specifiable Concerns, 45 

1.3.1.1 Agriculture and Water Resources, 45 

1.3.1.2 Rising Sea Level, 48 

1.3.2 More Speculative Concerns, 48 

1.3.3 The Problem of Unease about Changes of This 
Magnitude, 50 

1.4 POSSIBLE RESPONSES, 55 

1.4.1 Defining the Problem, 55 

1.4.2 The Organizing Framework, 57 

1.4.3 Categories of Response, 57 

1.4.4 Reprise, 61 

1.5 RECOMMENDATIONS, 61 

1.5.1 Can C0 2 Be Addressed as an Isolated Issue?, 61 

1.5.2 Actual and Near-Term Change of Policies, 62 

1.5.3 Energy Research and Policy, 64 

1.5.4 Synfuels Policy and C0 2 , 65 

1.5.5 Applied Research and Development, 66 

1.5.6 Basic Research and Monitoring, 67 

1.5.6.1 General Research Comments, 68 

1.5.6.2 The International Aspect, 70 



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1.5.6.3 Projecting CO 2 Emissions, 72 

1.5.6.4 Projecting CO 2 Concent rat ions , 72 

1.5.6.5 Climate, 74 

1.5.6.6 Detection and Monitoring, 76 

1.5.6.7 Impacts, 78 
1.6 CONCLUDING REMARKS, 81 
REFERENCES , 81 



2 FUTURE CARBON DIOXIDE EMISSIONS FROM FOSSIL FUELS 87 

2.1 FUTURE PATHS OF ENERGY AND CARBON DIOXIDE EMISSIONS, 87 
William D. Nordhaus and Gary W. Yohe 

2.1.1 Overview, 87 

2.1.2 Detailed Description of the Model, Data, and 
Results, 99 

2.1.2.1 The Model, 100 

2.1.2.2 The Data, 111 

2.1.2.3 Results, 130 
References, 151 

2.2 A REVIEW OF ESTIMATES OF FUTURE CARBON DIOXIDE EMISSIONS, 153 
Jesse H. Ausubel and William D. Nordhaus 

2.2.1 Introduction, 153 

2.2.2 Projections Based on Extrapolations, 155 

2.2.3 Energy System Projections, 156 

2.2.3.1 Perry-Lands berg (NAS) , 157 

2.2.3.2 IIASA, 157 

2.2.3.3 Rotty et al., 159 

2.2.3.4 Nordhaus, 160 

2.2.3.5 Edmonds and Re illy, 161 

2.2.3.6 Other Projections, 162 

2.2.4 Projections with C0 2 Feedback 
to the Energy System, 164 

2.2.4.1 Nordhaus, 164 

2.2.4.2 Edmonds and Re illy, 165 

2.2.4.3 CEQ, 166 

2.2.4.4 A. M. Perry et al., 167 

2.2.4.5 General Comments, 168 

2.2.5 A Note on the Biosphere, 169 

2.2.6 Projections of Non-C0 2 Trace Gases, 170 

2.2.7 Findings, 171 

2.2.7.1 The State of the Art, 171 

2.2.7.2 Likely Future Outcomes, 173 

2.2.8 Conclusion, 181 
References, 181 



xviii 



3 PAST AND FUTURE ATMOSPHERIC CONCENTRATIONS OF CARBON DIOXIDE 186 

3.1 INTRODUCTION Peter G. Brewer f 186 

3.2 CARBON DIOXIDE AND THE OCEANS Peter G. Brewer, 188 

3.2.1 Introduction, 188 

3.2.2 The Cycle of Carbon Dioxide within the Oceans, 189 

3.2.3 The Deep Circulation, 189 

3.2.4 Biological Activity, 190 

3.2.5 Deep Decomposition of Organic Matter, 191 

3.2.6 Calcium Carbonate, 191 

3.2.7 The Chemistry of C0 2 in Seawater, 194 

3.2.8 Measurements of Ocean CO 2 , 199 

3.2.9 Models of Ocean C0 2 Uptake, 202 

3.2.10 Future Studies and Problems, 209 

3.2.11 Summary, 211 
References, 211 

3.3 BIOTIC EFFECTS ON THE CONCENTRATION OF ATMOSPHERIC CARBON 
DIOXIDE: A REVIEW AND PROJECTION George M. Woodwell, 216 

3.3.1 Introduction, 216 

3.3.2 How Much Carbon is Held in the Biota and Soils?, 217 

3.3.2.1 The Biota, 217 

3.3.2.2 The Soils, 217 

3.3.2.3 Total Carbon Pool under Biotic 
Influences, 217 

3.3.3 Metabolism and the Storage of Carbon in Terrestrial 
and Aquatic Ecosystems, 218 

3.3.3.1 The Production Equation, 218 

3.3.3.2 A Basis in the Metabolism of Forests for 
the Oscillation in Atmospheric CO2 
Concentration, 219 

3.3.3.3 Factors Affecting Global Net Ecosystem 
Production, 221 

3.3.4 Changes in Area of Forests of the World, 225 

3.3.5 The Biota in the Context of the Global Carbon 
Balance, 229 

3.3.6 A Projection of Further Releases from Biotic 
Pools, 234 

3.3.7 Summary and Conclusions, 234 
References, 236 

3.4 THE ATMOSPHERE Lester Machta, 242 

3.4.1 Introduction, 242 

3.4.2 Changes in Atmospheric CO 2 Growth Rate with Time 
and Space, 243 

3.4.2.1 Shorter-Term Variation and Its Possible 
Cause, 243 

3.4.2.2 Longer-Term Variations, 246 

3.4.2.3 Change in Annual Cycle, 247 

3.4.2.4 Spatial Distribution, 248 

3.4.2.5 I so top ic Content of Atmospheric CO 2 , 249 

3.4.3 Conclusions, 250 
References, 250 

xix 



3.5 METHANE HYDRATES IN CONTINENTAL SLOPE SEDIMENTS AND INCREASING 
ATMOSPHERIC CARBON DIOXIDE Roger R. Revelle, 252 

3.5.1 Methane in the Atmosphere, 252 

3.5.2 Formation of Methane Clathrate in Continental Slope 
Sediments, 253 

3.5.3 Effect of Carbon Dioxide-Induced Warming on 
Continental Slope Clathrates, 256 

3.5.4 Future Rate of Methane Release from Sedimentary 
Clathrates, 257 

References, 260 

3.6 SENSITIVITY STUDIES USING CARBON CYCLE MODELS 
Lester Machta, 262 

3.6.1 Comparison among Different Models, 262 

3.6.2 Comparison of Parameters within a Single Model, 263 

3.6.3 Deforestation as a Source of CO 2 , 264 

3.6.4 Conclusion, 265 
References, 265 



4 EFFECTS ON CLIMATE 266 

4.1 EFFECTS OF CARBON DIOXIDE Joseph Smagorinsky, 266 

4.1.1 Excerpts from "Charney" and "Smagorinsky 11 
Reports, 266 

4.1.2 Epilogue, 277 
References, 282 

4.2 EFFECTS OF NON-C0 2 GREENHOUSE GASES Lester Machta, 285 
References, 291 



DETECTION AND MONITORING OF C0 2 -INDUCED CLIMATE CHANGES 292 

Gunter Weller, D. James Baker, Jr., W. Lawrence Gates, Michael 
C. MacCracken, Syukuro Manabe f and Thomas H. Vender Haar 

5.1 SUMMARY, 292 

5.2 HAVE C0 2 -INDUCED SURFACE TEMPERATURE CHANGES ALREADY 
OCCURRED?, 297 

5.2.1 Introduction, 297 

5.2.2 Requirements for Identifying C0 2 -Induced Climate 
Change, 301 

5.2.2.1 Climatic Data Bases, 303 

5.2.2.2 Causal Factors, 306 

5.2.2.3 Relating Causal Factors and Climatic Effects, 311 

5.2.3 Attempts to Identify C0 2 -Induced Climate Change, 313 

5.2.3.1 Carbon Dioxide as a Causal Factor, 318 

5.2.3.2 Volcanic Aerosol as a Causal Factor, 320 

5.2.3.3 Solar Variations as a Causal Factor, 322 

5.2.3.4 Combinations of Causal Factors, 324 

5.2.4 Steps for Building Confidence, 327 

5.3 A STRATEGY FOR MONITORING C0 2 -INDUCED CLIMATE CHANGE, 330 
5.3.1 The "Fingerprinting" Concept, 330 

xx 



5.3.2 Considerations in Climate Monitor ing , 331 

5.3.2.1 Statistical Variability and Expectations of 
Change, 331 

5.3.2.2 Initial Selection of Parameters, 332 

5.3.2.3 Revision and Application of a Monitoring 
Strategy, 332 

5.3.3 Candidate Parameters for Monitoring, 333 

5.3.3.1 Causal Factors, 335 

5.3.3.2 Atmospheric Parameters, 351 

5.3.3.3 Cryospheric Parameters, 361 

5.3.3.4 Oceanic Parameters, 367 

5.3.4 Conclusions and Recommendations, 370 

5.3.4.1 Priority of Parameters to be Monitored, 370 

5.3.4.2 Measurement Networks, 371 

5.3.4.3 Modeling and Statistical Techniques, 372 

5.3.4.4 Objective Evaluation of Evidence, 372 
REFERENCES, 373 

AGRICULTURE AND A CLIMATE CHANGED BY MORE CARBON DIOXIDE 383 
Paul E. Waggoner 

6.1 INTRODUCTION, 383 

6.1.1 Concentrating on a Critical, Susceptible, and 
Exemplary Subject, 383 

6.1.2 Agriculture and Past Changes in the Weather, 384 

6.1.3 The Range of Change in the Atmosphere, 386 

6.2 EFFECTS OF CO 2 ON PHOTOSYNTHESIS AND PLANT GROWTH, 388 

6.2.1 Photosynthesis, 388 

6.2.1.1 Rate of Photosynthesis, 389 

6.2.1.2 Duration of Photosynthesis, 390 ; 

6.2.1.3 Fate and Partitioning of Photosynthate, 390 I 

6.2.2 Drought, 391 ] 

6.2.3 Nutrients, 391 

6.2.3.1 Nitrogen Metabolism, 392 

6.2.3.2 Organic Matter and Rhizosphere 

Association, 392 ; 

6.2.4 Phenology, 393 

6.2.5 Weeds, 393 j 

6.2.6 Direct Effects of CO 2 on Yield, 394 ] 

6.3 PREDICTING THE CHANGES IN YIELD THAT WILL FOLLOW A CHANGE 1 
TO A WARMER, DRIER CLIMATE, 396 j 

6.3.1 History, 396 

6.3.2 Simulation, 403 

6.3.3 Summary, 405 

6.4 PATHOGENS AND INSECT PESTS, 405 

6.5 IRRIGATION IN A WARMER AND DRIER CLIMATE, 407 

6.6 ADAPTING TO THE CHANGE TO A WARMER, DRIER CLIMATE, 409 

6.6.1 Breeding New Varieties, 409 

6.6.2 Adapting to Less Water, 411 

6.7 CONCLUSION, 413 
REFERENCES, 413 

xx i 



7 EEPECTS OF A CARBON* DIOXIDE- INDUCED CLIMATIC CHANGE ON WATER 

SUPPLIES IN THE WESTERN UNITED STATES 419 

Roger R. Revelle and Paul E. Waggoner 

7.1 EMPIRICAL RELATIONSHIPS AMONG PRECIPITATION, TEMPERATURE f 
AND STREAM RUNOFF, 419 

7.2 EFFECTS OF CLIMATE CHANGE IN SEVEN WESTERN U.S. WATER 
REGIONS, 421 

7.3 THE COLORADO RIVER, 425 

7.4 CLIMATE CHANGE AND WATER-RESOURCE SYSTEMS, 431 
REFERENCES, 432 



PROBABLE FUTURE CHANGES IN SEA LEVEL RESULTING FROM INCREASED 
ATMOSPHERIC CARBON DIOXIDE 433 

Roger R. Revelle 

8.1 THE OBSERVED RISE IN SEA LEVEL DURING PAST DECADES, 433 

8.2 THE FUTURE RISE IN SEA LEVEL, 435 

8.2.1 Melting of Greenland Ice Cap and Alpine Glaciers, 436 

8.2.2 Heating of the Upper Oceans, 437 

8.2.3 Possible Disintegration of the West Antarctic Ice 
Sheet, 441 

REFERENCES, 447 



CLIMATIC CHANGE: IMPLICATIONS FOR WELFARE AND POLICY 449 

Thomas C. Schelling 

9.1 INTRODUCTION, 449 

9.1.1 Uncertainties, 451 

9.1.2 The Time Dimension, 451 

9.1.3 Discounting, Positive or Negative, 452 

9.1.4 Perspective on Change, 453 

9.1.5 Prudential Considerations, 454 

9.1.6 Variation in Human Environments, 455 

9.2 A SCHEMA FOR ASSESSMENT AND CHOICE, 456 

9.2.1 Five Categories, 463 

9.2.2 Background Climate and Trends, 465 

9.2.3 Production of C0 2 f 466 

9.2.4 Removal of CO 21 467 

9.2.5 Modification of Climate and Weather, 468 

9.2.6 Adaptation, 470 

9.2.7 Breathing C0 2 , 471 

9.2.8 Change in Sea Level, 472 

9.2.9 Defenses against Rising Sea Level, 472 

9.2.10 Food and Agriculture, 474 

9.2.11 Global Warming and Energy Consumption, 476 

9.2.12 Distributional Impact, 477 

9.3 SUMMING UP, 477 



xxii 



Annex 1: Report of Informal Meeting on C02 and the 

Arctic Ocean 483 

Roger R. Revelle 

Annex 2: Historical Note 488 

Jesse H. Ausubel 

Annex 3: Energy Security Act of 1980 492 

Annex 4: Background Information on Committee Members 494 



xxiii 



Executive Summary 



1. Carbon dioxide (C0 2 ) is one of the gases of the atmosphere 
important in determining the Earth's climate. In the last generation 
the C02 concentration in the atmosphere has increased from 315 parts 
per million (ppm) by volume to over 340 ppmv. (Chapters 3, 4) 

2. The current increase is primarily attributable to burning of 
coal, oil, and gas; future increases will similarly be determined 
primarily by fossil fuel combustion. Deforestation and land use 
changes have probably been important factors in atmospheric C02 
increase over the past 100 years. (Chapters 2, 3) 

3. Projections of future fossil fuel use and atmospheric concen- 
trations of CC>2 embody large uncertainties that are to a considerable 
extent irreducible. The dominant sources of uncertainty stem from our 
inability to predict future economic and technological developments 
that will determine the global demand for energy and the attractiveness 
of fossil fuels. We think it most likely that atmospheric CO 2 con- 
centration will pass 600 ppm (the nominal doubling of the recent level) 
in the third quarter of the next century. We also estimate that there 
is about a l-in-20 chance that doubling will occur before 2035. 
(Chapters 2, 3) 

4. If deforestation has been a large net source of CO 2 in recent 
decades, then the models that we are using to project future atmospheric 
concentrations are seriously flawed; the fraction of man-made C0 2 
remaining airborne must then be lower, and C0 2 increase will probably 
occur more slowly than it otherwise would. (Chapter 3) 

5. Estimates of effects of increasing C0 2 on climate also embody 
significant uncertainties, stemming from fundamental gaps in our under- 
standing of physical processes, notably the processes that determine 
cloudiness and the long-term interactions between atmosphere and 
ocean. (Chapter 4) 

6. Several 'other gases besides C0 2 that can affect the climate 
appear to be increasing as a result of human activities; if we project 



increases in all these gases , climate changes can be expected sig- 
nificantly earlier than if we consider C0 2 alone. (Chapter 4) 

I. Prom climate model simulations of increased CO 2 we conclude 
with considerable confidence that there would be global mean 
temperature increase. With much less confidence we infer other more 
specific regional climate changes f including relatively greater polar 
temperature increase and summer dryness in middle latitudes '(e.g., the 
latitudes of the United States) . (Chapter 4) 

8. Results of most numerical model experiments suggest that a 
doubling of C0 2 , if maintained indef initely, would cause a global 
surface air warming of between 1.5C and 4.5C. The climate record of 
the past hundred years and our estimates of C02 changes over that 
period suggest that values in the lower half of this range are more 
probable. (Chapters 4, 5) 

9. By itself f C0 2 increase should have beneficial effects on 
photosynthesis and water-use efficiency of agricultural plants, 
especially when other factors are not already limiting growth. 
(Chapters 3, 6) 

10. Analysis of the effects of a warmer and drier climate on rain- 
fed agriculture in the United States suggests that over the next couple 
of decades negative effects of climate change and positive effects from 
C0 2 fertilization both will be modest and will approximately balance. 
The outlook is more troubling for agriculture in lands dependent on 
irrigation. Longer-term impacts are highly uncertain and will depend 
strongly on the outcome of future agricultural research, development, 
and technology. (Chapter 6) 

II. Changes in temperature and rainfall may be amplified as changes 
in the annual discharge of rivers. For example, a 2C warming could 
severely reduce the quantity and quality of water resources in the 
western United States. (Chapter 7) 

12. (a) If a global warming of about 3 or 4C were to occur over the 
next hundred years, it is likely that there would be a global sea- level 
rise of about 70 cm, in comparison with the rise of about 15 cm over 
the last century. More rapid rates could occur subsequently, if the 
West Antarctic Ice Sheet should begin to disintegrate. (Chapter 8) 

(b) Such a warming might also bring about changes in Arctic ice 
cover, with perhaps a disappearance of the summer ice pack and asso- 
ciated changes in high-latitude weather and climate. (Annex 1) 

13. Because of their large uncertainties and significant implica- 
tions, it is important to confirm the various predictions of climate 
changes at the earliest possible time and to achieve greater precision. 
This can best be done through carefully designed monitoring programs of 
long duration emphasizing the ensemble of variables believed to 
influence climate or to reflect strongly the effect of C0 2 . (Chapter 5) 



14. The social and economic implications of even the most carefully 
constructed and detailed scenarios of C0 2 increase and climatic con- 
sequences are largely unpredictable. However, a number of inferences 
seem clear: 

(a) Rapid climate change will take its place among the numerous 
other changes that will influence the course of society, and these 
other changes may largely determine whether the climatic impacts of 
greenhouse gases are a serious problem. 

(b) As a human experience, climate change is far from novel? 
large numbers of people now live in almost all climatic zones and move 
easily between them. 

(c) Nevertheless, we are deeply concerned about environmental 
changes of this magnitude; man-made emissions of greenhouse gases 
promise to impose a warming of unusual dimensions on a global climate 
that is already unusually warm. We may get into trouble in ways that 
we have barely imagined, like release of methane from marine sediments, 
or not yet discovered. 

(d) Climate changes, their benefits and damages, and the 
benefits and damages of the actions that bring them about will fall 
unequally on the world's people and nations. Because of real or 
perceived inequities, climate change could well be a divisive rather 
than a unifying factor in world affairs. (Chapter 9) 

15. Viewed in terms of energy, global pollution, and worldwide 
environmental damage, the "C02 problem" appears intractable. Viewed 

as a problem of changes in local environmental factors rainfall, river 
flow, sea level the myriad of individual incremental problems take 
their place among the other stresses to which nations and individuals 
adapt. It is important to be flexible both in definition of the issue, 
which is really more climate change than C(>2, and in maintaining a 
variety of alternative options for response. (Chapter 9) 

16. Given the extent and character of the uncertainty in each segment 
of the argument emissions, concentrations, climatic effects, environ- 
mental and societal impacts a balanced program of research, both basic 
and applied, is called for, with appropriate attention to more signifi- 
cant uncertainties and potentially more serious problems. (Chapter 1) 

17. Even very forceful policies adopted soon with regard to energy 
and land use are unlikely to prevent some modification of climate as a 
result of human activities. Thus, it is prudent to undertake applied 
research and development and to consider some adjustments in regard 
to activities, like irrigated agriculture, that are vulnerable to 
climate change. (Chapters 1, 9) 

18. Assessment of the C0 2 issue should be regarded as an iterative 
process that emphasizes carry over of learning from one effort to the 
next. (Chapter 1) 

19. Successful response to widespread environmental change will be 
.facilitated by the existence of an international network of scientists 



conversant with the issues and of broad international consensus on facts 
and their reliability. Sound international research and assessment 
efforts can turn up new solutions and lubricate the processes of change 
and adaptation. (Chapter 1) 

20. With respect to specific recommendations on research, develop- 
ment, or use of different energy systems, the Committee offers three 
levels of recommendations. These are based on the general view that, 
if other things are equal, policy should lean away from the injection 
of greenhouse gases into the atmosphere. 

(a) Research and development should give some priority to the 
enhancement of long-term energy options that are not based on com- 
bustion of fossil fuels. (Chapters 1, 2, 9) 

(b) We do not believe, however, that the evidence at hand about 
CO 2" induced climate change would support steps to change current 
fuel-use patterns away from fossil fuels. Such steps may be necessary 
or desirable at some time in the future, and we should certainly think 
carefully about costs and benefits of such steps; but the very near 
future would be better spent improving our knowledge (including knowl- 
edge of energy and other processes leading to creation of greenhouse 
gases) than in changing fuel mix or use. (Chapters 1, 2, 9) 

(c) It is possible that steps to control costly climate change 
should start with non-C0 2 greenhouse gases. While our studies 
focused chiefly on C0 2 , fragmentary evidence suggests that non-C0 2 
greenhouse gases may be as important a set of determinants as C0 2 
itself. While the costs of climate change from non-C0 2 gases would 

be the same as those from C0 2 , the control of emissions of some 
non-C0 2 gases may be more easily achieved. (Chapters 1, 2, 4, 9) 

21. Finally, we wish to emphasize that the C0 2 issue interacts 
with many other issues, and it can be seen as a healthy stimulus for 
acquiring knowledge and skills useful in the treatment of numerous 
other important problems. (Chapter 1) 



1 Synthesis 

Carbon Dioxide Assessment Committee 



1.1 INTRODUCTION 

For more than a hundred years scientists have been suggesting that 
slight changes in the chemical composition of the atmosphere could 
bring about major climatic variations. Since the turn of the century, 
the focus has been particularly on worldwide release of carbon dioxide 
(C0 2 ), as a result of burning of coal f oil, and gas and changes in 
land use that release C0 2 from forests and soils.* In recent decades 
many aspects of the argument that enough C0 2 will be released to 
bring about unwanted and unwonted changes in climate have been filled 
out and strengthened; at the same time, new questions about segments of 
the argument have arisen, and possible benefits have been identified, 
including directly favorable implications for plant growth from 
increasing CO 2 . 

At this stage in the history of the C0 2 question, many readers are 
familiar with its basic aspects, so we have limited this introduction 
to two fundamental points. The first is that C0 2 , along with water 
vapor, ozone, and a variety of other compounds, is a key factor in ,| 

determining the thermal structure of the atmosphere. These so-called fi j 

"greenhouse" gases do not strongly absorb incoming radiation for most f j 

of the shortwave solar spectrum, but they are more effective absorbers ! |J 

of the long-wavelength (infrared) radiation of the Earth's surface and * j 

atmosphere (see Figure 1.1). The mix and distribution of the gases ^ 

account in no small part for the generally hospitable climate of Earth 
and the inhospitable climate of other planets. Concern arises about 
human activities that release greenhouse gases because important 
absorption bands for C0 2 and other atmospheric gases are far from 
saturation; increasing the concentration of the gases will continue to 
affect the net emission or absorption of energy from a given layer of 
the atmosphere and thus the climate. The second fundamental point is 
that the atmospheric concentration of C0 2 is rising. Figure 1.2 
shows an exceptionally accurate and reliable record of measurements 



*See "Annex 2, Historical Note," for the early history of the C0 2 
issue. 



Wavelength (Aim) 

15 10 

"1 




FIGURE 1.1 Infrared spectrum of the Earth as taken by a scanning 
interferometer on board the Nimbus- 4 satellite over the North African 
desert. Also shown (dotted lines) are the blackbody radiances that 
would be observed at various temperatures. Thus, in the 10-13-ym 
region, the atmosphere is transparent and the radiance corresponds 
closely to that expected from the hot desert surface (at 320 K or 
47C). In the C0 2 band, however, the radiance is from the 
stratosphere at a temperature of 220 K, and energy from the Earth's 
surface is blocked. Other important infrared-absorbing trace gases 
include water vapor, nitrous oxide, methane, the chlorofluorocarbons, 
and ozone (in the troposphere) . (From Paltridge and Platt, 1976, after 
Hanel et al., 1972). 



starting in 1958. In short, there is a strong physical basis for 
attention to the 0)2 question in both theory and measurement. 

Another, quite different, aspect of the C0 2 issue that requires 
introduction is that the time horizons of the subject and, therefore, 
of this report are very long. We talk about American agriculture in 
the year 2000, global energy use to 2100, and possible changes in sea 
level over the next three to five centuries. Is it meaningful to talk 
of such remote times? We think it is, and we have tried to devise 
approaches that take the time dimension seriously, although much of the 
report had to be speculative. Is it necessary to look so far into the 
future? Again, we think the answer is yes. Once the C0 2 content of 
the atmosphere rises significantly, it is likely to remain elevated for 
centuries; so from a physical point of view one must consider the long 
run. From the perspective of human activities, the time periods to be 
considered are also necessarily long. It takes many decades to replace 
the capital and infrastructure associated with a particular form of 
energy, and the time to develop large-scale water supply systems can be 
equally great. No policy, no matter how forceful, will make the issue 
of climate change disappear for at least decades to come. Finally, in 
considering the environment, one must think in terms of long-term 



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8 

sustainable strategies. While adverse consequences of 100 years from 
now are obviously less pressing than those of next year/ if they are 
also of large magnitude and irreversible/ we cannot in good conscience 
discount them. 

The outline of the report is as follows. In this Synthesis chapter 
we summarize the outlook for CO 2 - induced climatic change and its 
effects, try to estimate how serious an issue CO 2 is/ and make 
recommendations for improving our understanding of the issue and our 
societal stance in regard to it. In subsequent chapters, individual 
authors and groups of authors treat the same topics in greater depth 
and for more specialized audiences. Both the Synthesis and the volume 
as a whole examine this sequence of questions: How much CC>2 will be 
emitted? How much will remain in the air? How much will C02 and 
other greenhouse gases change the climate? Are CC^-induced climatic 
changes already identifiable? What would be the effects of substantial 
warming induced by increased atmospheric concentrations of C0 2 and 
other greenhouse gases on agriculture, water supply, and polar regions 

and sea level? And, finally, what are the implications of the C02 
issue for societal welfare and policy? Figure 1.3 offers an overview 
of the C(>2 issue as treated in this report. 




Use of 
Fossil 
Fuels 

2.1J.2 



Deforest- 
ation 
a*d 
Land Use 

2.2,3.3 



C0 ? 
Emissions 

2,3 




Adapt, Compensate, Reduce 
Vulnerability, Absorb Impacts 

6,9 



Agriculture 
6 



Water 
6,7 



Sea Level/ 
Polar Regions 

8,9 










Policy 


Choices 


(Actions) 


9 




Research, 


Applied 


Research 


and 




Development 


1 




Monitor 


5 





FIGURE 1.3 An overview of the C0 2 issue. 
or sections that focus on topic in box. 



T 



Numbers refer to chapters 



1.2 THE OUTLOOK 

1.2.1 Future C0 9 Emissions 

By far the largest potential sources of man-made CO 2 emissions are 
the fossil fuels, especially the abundant supplies of coal. Current 
annual fossil fuel emissions are estimated at about 5 x 10 9 tons of 
carbon (Gt of C) + 10% (Mar land and Rotty, 1983) . In 1981, emissions 
came about 44% from oil, 38% from coal, and 17% from gas. The United 
States accounts for about one quarter of worldwide fossil fuel emis- 
sions, as do Western Europe and Japan, the Soviet Union and Eastern 
Europe, and developing countries. 

To estimate future emissions of CO 2 from fossil fuels , Nordhaus 
and his co-authors adopted two approaches. One was to review previous 
global, long-range energy studies and use the range of projections as a 
guide to the uncertainty of scientific judgment (Ausubel and Nordhaus, 
this volume, Chapter 2, Section 2.2). The second approach (Nordhaus 
and Yohe, this volume, Chapter 2, Section 2.1), developed for this 
assessment, explicitly allows estimation of future emissions and their 
uncertainty based on a range of values for key parameters. 

Review of previous energy studies shows that almost all studies 
applicable to estimation of C0 2 emissions project a continued marked 
growth of energy demand. For example, projections of energy demand in 
the year 2030 generally range from about 2-1/2 to 5 times the recent 
rate of energy use of 8 terawatt (TW, 10 12 W) years per year. The 
studies vary so widely in quality, approach, level of detail, time 
horizon, data base, and geographic aggregation that strict comparisons 
are generally inappropriate. However, some generalizations may be 
ventured. Most studies looking beyond the year 2000 project average 
energy growth between about 2% and slightly above 3% per year, rates 
reasonably consistent with the 2.2% global annual average increase in 
primary energy consumption* that has prevailed over the past 120 
years. Of course, the absolute range of projections spreads as the 
time horizon is extended, as a result of compounding the varied annual 
rates of increase. To illustrate, the range embracing almost all of 
the more detailed projections increases from 14-21 TW yr/yr in A.D. 
2000 to 20-40 TW yr/yr a generation later. There are no strong signs 
of convergence toward a single, widely accepted projection or set of 
assumptions, although generally estimates have been lower in the last 
few years than in the 1970s. Figure 1.4 summarizes past global energy 
consumption and most of the long-range projections. 

Combining estimates of energy demand and the mix of fuels leads to 
projections of C0 2 emissions. When the mix includes a large share of 
fossil energy, the projections show relatively high levels of C0 2 
emissions. Figure 1.5 shows paths of C0 2 emissions derived from 
about a dozen long-range energy projections. Average annual rates of 



*The 2.2%/yr figure includes wood and noncommercial energy sources 
(Marchetti and Nakicenovic, 1979; Nakicenovic, 1979). 



10 



60, 


* 


i 

Edmonds and Reilly (1983) / 




* 


Exxon (1980) / 


50 





NASA (1981) / 




A 


Nordhaus (1977) / 







OECD-lnterfutures (1979) f 




m 


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40 





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/ 7+ 




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1900 1920 1940 1960 1970-74 1980-82 2000 20 25-30 



1500 



1200 



900 



600 



300 



-Jo 



YEAR 

FIGURE 1.4 Past and projected global energy consumption. See Chapter 
2, Section 2.2 for discussion of projections included here. 



increase in C0 2 emissions to 2030 generally range from about 1 to 
3.5%.* Estimated annual emissions range between about 7 and 13 Gt of C 
in the year 2000 and f with a few exceptions , between about 10 and 30 Gt 
of C in 2030. Thus f based on a review of past efforts, one might infer 
that energy consumption 50 years hence could differ by at least a factor 
of 2 and associated C0 2 emissions by a factor of 3 or more. 

For purposes of understanding future outcomes and weighing policy 
choices , the past efforts reviewed leave open important questions. 
They generally do not allow a judgment as to the accuracy with which a 
forecast is made. It is of central importance in many policy problems 
to know not only the best judgment about an event (such as the time 
when atmospheric C0 2 will pass a certain level) but also to be able 
to estimate the degree of precision or approximation about that judg- 
ment. Some studies have approached the difficulties of forecasting by 



*This range contrasts with the 4.3% figure for past and projected 
growth in C02 emissions that prevailed for several years in the 
literature on the C0 2 issue. The mean growth rate of fossil fuel 
C0 2 emissions over the past 120 years has more recently been 
estimated at about 3.5% per year (Elliott, 1983). 



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12 

use of "scenario" analysis. The scenario approach involves tracing out 
time paths for important parameters under assumptions that are thought 
to be " interesting ," usually without assigning measures of probability 
to the parameters or outcomes. These studies provide answers to hypo- 
thetical questions of the "what if?" type. For example, What might be 
the evolution of the energy system if there is a moratorium on building 
nuclear power plants? Usually scenario studies examine only a very few 
possibilities, and they do not attempt to assess the actual likelihood 
of the scenarios investigated. 

To address these shortcomings, Nordhaus and Yohe employed modern 
developments in aggregative energy and economic modeling to construct a 
simple model of the global economy and carbon dioxide emissions. Par- 
ticular care was given to assure that the energy and production sectors 
of the economy were integrated (most C0 2 emission projections are 
based on examination of the energy sector taken in isolation) and to 
respect the cost and availability of fossil fuels. The analysis' 
attempts to recognize explicitly the intrinsic uncertainty about future 
developments by identifying the most important uncertain parameters of 
the model, by examining current knowledge and disagreement about these 
parameters, and then by specifying a range of possible values for each 
uncertain parameter. The emphasis was not to resolve uncertainties but 
to represent current uncertainties as realistically as possible. A use 
of the range of paths and uncertainties for the major economic, energy, 
and carbon dioxide variables allows not only a "best guess" of the 
future path of carbon dioxide emissions but also alternative trajec- 
tories that represent a reasonable range of possible outcomes given the 
current state of knowledge. The data employed were gathered from 
diverse sources and are of quite different levels of precision; judg- 
ments as to the uncertainties about the parameters are rough. Political 
conditions are not treated explicitly, but they may be regarded as 
included implicitly, for example, as a possible cause of a low value 
for the parameter representing growth in productivity. 

The central tendency in the results of the Nordhaus-Yohe approach is 
a lower emissions rate than that of most earlier studies, in which the 
annual emissions increase generally ranged from about 1 to 3.5%. The 
"best guess" of Nordhaus and Yohe is that C0 2 emissions will grow at 
about 1.6% annually to 2025, then slow their growth to slightly under 
1% annually after 2025. The major reasons for the lower rate are a 
slower estimated growth of the global economy than had earlier been the 
general assumption, further conservation as a result of the energy price 
increases of the past deacde, and a tendency to substitute nonfossil 
for fossil fuels as a result of the increasing cost of fossil fuels 
relative to other fuels. Figure 1.6 presents five paths that represent 
the 5th, 25th, 50th ("best guess"), 75th, and 95th percentiles of 
annual C0 2 emissions. The percentiles are indexed in terms of the 
cumulative C0 2 emissions by the year 2050 (see this volume, Chapter 
2, Section 2.1) . 

In addition to burning of fossil fuels, human activities release 
C0 2 through deforestation and land clearing. Estimates of future 
biospheric emissions have generally been based on extrapolation of 
estimates of recent biotic emissions and rough guesses about what 



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1975 2000 2025 2050 

YEAR 



2075 



2100 



FIGURE 1.6 Carbon dioxide emissions from a sample of 100 randomly 
chosen runs. The 5th r 25th, 50th, 75th, and 95th percentile runs for 
yearly emissions, with emissions for years 2000, 2025, 2050, and 2100 
indicated. See Chapter 2, Section 2.1, and Figure 2.17 for further 
detail. 



proportion of carbon in the biosphere might be subject to human 
influence. Models of the carbon in forests and soils are now being 
developed that might provide projections with a stronger theoretical 
basis. An estimate for the maximum possible future addition from all 
biospheric sources is 240 Gt of C (Revelle and Munk, 1977), and Woodwell 
(this volume, Chapter 3, Section 3.3) offers a similar projection. 
Baumgartner (1979) estimates that clearing of all tropical forests might 
contribute about 140 Gt of C. The total carbon content of the Amazon 
forest is estimated at about 120 Gt of C (Sioli, 1973) . Chan et al. 
(1980) develop a high deforestation scenario in which total additional 
transfer of carbon from the biosphere to the atmosphere by the year 
2100 is about 100 Gt of C. The World Climate Programme (1981) group of 
experts adopted a range of 50 to 150 Gt of C for biospheric emissions 
in the 1980 to 2025 period. Projections of future atmospheric C0 2 



14 

concentrations embracing both burning of fossil fuels and terrestrial 
sources have all been dominated by growth rates in fossil fuel emis- 
sions, except in cases where fossil fuel emissions are extremely low. 
Over a period of a decade or two, biospheric emissions could rival 
fossil fuel emissions, but over a century biospheric emissions from 
human activities are most unlikely to average higher than 1 to 3 Gt of 
C per year, and fossil fuel emissions are typically projected to be an 
order of magnitude larger. It is also possible, as discussed in the 
next section, that there will be no significant net release of carbon 
from the biosphere over the next century, depending on which human 
activities and physical processes are dominant. 



1.2.2 Future Atmospheric C02 Concentrations 

We now turn to the question of translating CO 2 emissions into atmo- 
spheric C0 2 concentrations. Projecting C0 2 concentrations requires 
a determination of how emissions will be partitioned among the atmo- 
sphere, the oceans, and the biosphere. Carbon circulates naturally 
among these reservoirs driven by physical and biological forces; we 
term this circulation the carbon cycle, and the injection of carbon 
dioxide into the atmosphere by human activities may be viewed as a 
perturbation of this cycle (see Figure 1.7). Before the substantial 
release of C0 2 from human activities began, one part of the carbon 
cycle involved production of organic matter from atmospheric C0 2 and 
water and transportation of this material to the ocean where it was 
buried in marine sediments. Anthropogenic emissions have reversed this 
part of the carbon cycle. Some of the carbon stored over 500 million 
years in marine sediments is now returning to the atmosphere in a few 
short generations. 

By means of quantitative models of the carbon cycle, we can estimate 
the effects of postulated rates of carbon dioxide injections on the 
pools and fluxes of carbon, in particular on the concentration of 
carbon in the atmosphere. Moreover, we can also assess the degree to 
which uncertainties in our understanding of one or another factor 
influence our forecasts of future atmospheric C0 2 levels. 

The atmosphere forms a thin film over the Earth. Its composition is 
in large part the result of biological activity, and it is powerfully 
shaped by interaction with the oceans that cover some 70% of the 
globe. The concentration of C0 2 in the atmosphere has varied over 
the ages; for example, there is evidence that it may have been about 
200 ppm during the last ice age 18,000 years ago. Reliable instrumental 
records are available only since 1957. Since then, the concentration 
has increased from about 315 ppm to slightly above 340 ppm and at an 
average rate of about 0.4% per year over the last decade (Figure 1.2). 
As discussed by Machta (this volume, Chapter 3, Section 3.4), carbon 
dioxide is well mixed in the atmosphere; measurements from the global 
network of sampling sites show relatively small spatial and temporal 
variations that are explainable largely in terms of fossil fuel sources 



15 



(increasing 2-3/yr in 
recent years) 



FORESTS AND TERRESTRIAL BIOTA 




Organic carbon 100 s 
Inorganic carbon 10,000 s 



Oil-100s 

Gas-100s 

Oil shale and deep gas-1000 s 

Coal -1000s 



Clathrates in continental 
slope sediment 

-10,000? 



FIGURE 1.7 Some global carbon pools and annual fluxes. Estimated 
sizes of pools and fluxes are in gigatons of carbon. Estimates are 
rounded from figures given in Chapters 2 and 3 and from Clark, 1982. 
Pools that are only broadly measured are assessed here in order of 
magnitude, e.g., hundreds (100s). Dashed arrows represent additional 
fluxes due to human activities. 



and biological, atmospheric, and oceanic processes, such as the annual 
cycle of the terrestrial biota and the quasi-periodic Southern 
Oscillation. Recently noted increases in the amplitude of the annual 
cycle may be related to changes in the cycling of CO2 through photo- 
synthesis and respiration. The year-to-year increases in atmospheric 
concentrations are generally becoming larger with time, roughly in step 
with emissions of CO 2 from fossil fuel combustion. Indeed, according 
to Machta (this volume, Chapter 3, Section 3.4), most of the atmospheric 
variations during the past 20 years are more easily accounted for when 
we consider a growing fossil fuel source alone than when we consider 
any significant additional source, such as deforestation* 

Comparison of fossil fuel C0 2 releases and growth in atmospheric 
concentrations shows that a quantity somewhat larger than half the 
fossil fuel CO 2 has remained in the atmosphere. The rest must have 
been transferred to some other reservoir, primarily to the ocean. As 
described by Brewer (this volume. Chapter 3, Section 3.2), the capacity 
of the ocean as a sink for C0 2 is a function of its chemistry and 
biology, and the rate at which its capacity can be brought into play is 
a function of its physics. The low- and mid-latitude oceans are stably 



16 

stratified, capped by a warm surface layer that is approximately in 
equilibrium with atmospheric C0 2 . The deep waters of the world's 
oceans are formed in polar seas and slowly circulate through the ocean 
basins f with an abyssal (deep ocean) circulation time estimated at 
about 500 years. In the surface waters, CO 2 is fixed by photosyn- 
thetic activity and rapidly cycled by grazing organisms, with slow 
sedimentation of carbon into deeper layers where oxidation or deposition 
takes place. The absorption of C0 2 by the ocean is buffered by reac- 
tions with dissolved carbonate and bicarbonate ions. In the surface 
mixed layer, the "buffer factor" increases with growing C0 2 concen- 
trations, and the capacity of the ocean to absorb C0 2 added to the 
atmosphere will decrease unless additional factors change. Measurements 
of ocean C0 2 at the surface and at depth are consistent with our 
understanding of the processes involved and confirm the observed 
atmospheric increases in concentration. Figure 1.8 shows the increase 
in C0 2 in the atmosphere and in surface ocean waters since 1957. 

Mathematical models of ocean C0 2 uptake have been able to reproduce 
the records of C0 2 and related observations that we have, although 
complex processes of vertical transport have been modeled as simple 
diffusion. Radionuclides injected into the atmosphere by bomb tests 
have served as effective tracers of ocean circulation and essential 
empirical calibrators of these ocean models. However, the expression 
of all oceanic physics as a single unrealistic process hardly inspires 
confidence in the models' ability to deal with altered climatic regimes 
in the future. Moreover, potentially significant processes, such as 
changing riverine fluxes of terrestrial carbon and nutrients to the 
sea, may not yet have been adequately evaluated and represented. 
Nevertheless, at least for the short run, present-day models appear 
satisfactory to answer the question of how much anthropogenic C0 2 is 
taken up by the ocean. Brewer (this volume, Chapter 3, Section 3.2) 
reports several estimates that give a value at the present time of 
about 2 Gt of C/yr, or 40% of fossil fuel emissions. 

The terrestrial biota and soils contain about three times as much 
carbon as the atmosphere, and their changes could influence the atmo- 
spheric burden. The most active and vulnerable portion of the biota is 
in forests, which probably contain between about 260 and 500 Gt of C 
(Olson and Watts, 1982) . As discussed by Woodwell (see Figure 1.9 and 
Chapter 3, Section 3.3) , the net flux of carbon between the atmosphere 
and any ecosystem depends on the balance between photosynthetic produc- 
tion by green plants and respiration by both plants and other organisms. 
Woodwell observes that on land photosynthesis is more susceptible to 
disturbance than respiration, so disturbance of the biota tends to 
release carbon into the atmosphere in the period following disturbance. 
Subsequently, over a period of years to decades or longer, the balance 
may shift through recovery due to succession and the slow migration of 
plants in response to a new, stable environment. Increased C0 2 
enhances photosynthesis but does not necessarily lead to increased 
storage of carbon; on the other hand, increased temperature tends to 
increase respiration. Extension of growing seasons and extension or 
reduction of biomes with climate change can also affect the terrestrial 
carbon balance, particularly on longer time scales. It is thus 



17 



tr 

350 



UJ 

a 



z ~ 

~ a 300 
E a. 



7 O 

2 2 

CJ 

O 

o 

Q. 



250 



- T --p--| i 



1 i ' "- t=r ~r 



ATMOSPHERIC C0 2 




TTO/NA 



1.5 0.5/iatm/yr 



IGY 



r- 
m 



J _1 !._.! 1 I 


CD 
CD 



I 1 I 



in 
u> 

CD 



o 

I s - 



I I I 1 I 

m 
01 



o 

00 



YEAR 



FIGURE 1.8 Mean Sargasso gyre surface water pC0 2 versus time. The 
atmospheric C0 2 concentration is expressed in mole fraction C0 2 in 
dry air at Mauna Loa, Hawaii. The IGY was the International Geophysical 
Year; GEOSECS is the Geochemical Ocean Sections Study; TTO/NA refers to 
the program of observation of transient tracers in the North Atlantic. 
(Source: Takahashi et al., 1983.) See Brewer, Chapter 3, Section 3.2 
for further explanation. 



plausible that future changes in the atmosphere could lead to a 
significantly increased net biotic flux of carbon to or from the 
atmosphere. 

The possible sources of biotic C0 2 emissions deforestation and 
land disturbance are poorly documented. There are several indirect 
approaches to estimating what this biotic contribution may have been, 
largely based on correlation between growth in human population and the 
rate of conversion of forest for agriculture. The main direct data 
sources on deforestation have been the production yearbooks of the Pood 
and Agriculture Organization (FAO) of the United Nations , published 
since 1949 , and these may be inaccurate. All examinations of the biota 
conclude that there has been a marked reduction in storage of carbon 
over the past century or so; however, the timing and amount are the 



18 



FACTORS AFFECTING C0 2 IN AIR 



O 



LL 
O 
CO 



CC 
(D 




1360 



- 350 



- 340 



I 



- 330 8 



11/76 



MAM 



N 



FIGURE 1.9 The upper graph shows the course of total respiration and 
gross photosynthesis of an oak-pine forest in central Long Island, New 
York. Integration of these two curves produced the prediction of the 
annual change in local atmospheric C0 2 shown in the lower graph. The 
amplitude predicted in this way was considerably greater than observed 
(Woodwell et al., 1973), apparently because of mixing with air from 
over the oceans. Fossil fuel emissions are included for comparison. 



19 

subject of substantial dispute. Based on a model of the terrestrial 
carbon cycle, Woodwell (this volume, Chapter 3, Section 3.3) reports a 
range of 1.8 to 4.7 Gt of C per year at present. Other researchers 
have suggested lower figures (see Table 3.3). All but the lowest 
values in Woodwell 1 s range are impossible to reconcile with atmospheric 
observations and present-day ocean models. A net biotic carbon source 
of about 2 Gt/yr suggests that about 40% of all (fossil fuel plus 
biotic) CO 2 emissions have been remaining in the atmosphere, whereas 
a net biotic source near zero suggests about 60% of all anthropogenic 
CO 2 emissions remain airborne. 

There is also disagreement about whether significant regrowth of 
forests in some areas and stimulation of plant growth ("fertilization") 
by increased atmospheric C0 2 are taking place, perhaps countering 
losses from deforestation. While the increasing amplitude of the 
seasonal cycle may be interpreted as an indication of regrowth and 
stimulation, there is as yet no direct evidence of regrowth and stimu- 
lation of net ecosystem storage of carbon sufficient to balance the 
apparent effects of deforestation. At the plant level, there are 
arguments for increased growth; but at the community level, especially 
in forests, the relevant mechanisms may be limited by other factors 
(see Chapter 3, Section 3.3, and Chapter 6). Growth of much biomass 
may be limited by availability of land, solar radiation, water, and 
nutrients other than carbon. Inventories of biota and other measure- 
ments (e.g., width of tree rings) are not sufficient at present to 
resolve the debate. 

Finally, there is dispute about the level of C0 2 in the atmosphere 
in the last century before anthropogenic sources began to raise it. 
The preindustrial (circa 1850) concentration probably lay in the range 
250-295 ppm, with 260-280 ppm a preferred interval (see Machta, this 
volume, Chapter 3, Section 3.4). Backward extrapolation of contemporary 
observations based solely on estimated fossil fuel CO 2 injections and 
a constant airborne fraction leads to an estimate of about 290 ppm at 
the turn of the century. Chemical and other measurements made at that 
time span a range about the same value. Inferences from air trapped in 
glacial ice and from deep-ocean measurements indicate concentrations in 
the vicinity of 265 ppm in the middle of the last century. The dis- 
crepancy between mid- and late-nineteenth century data, if real, might 
partly be accounted for by emissions from the terrestrial biosphere. 

For the period since about 1950, we have records of C0 2 emissions 
from fossil fuels that are reliable within 10-15%. For the period 
before this, the record of fossil fuel emissions is less reliable. 

In view of the uncertainty and conflicting evidence about the 

preindustrial concentration, fossil fuel emissions, the biotic con- 

tribution, and ocean uptake, one reasonable approach is to use models 
to project different outcomes based on different assumptions about the 
various reservoirs and mechanisms (see Machta, this volume, Chapter 3, 

Section 3.6). In the last few years, carbon cycle models, like energy j 

models and climate models, have become more sophisticated. There are 
now several dynamic, process-oriented models that are making progress 
in representing, for example, accumulation and decay of dead vegetation? 
processing of carbon in soils and humus; and the biology, chemistry, j 



20 

and physics of the oceans. Published models have been calibrated to 
agree well with what is known about recent CO 2 trends, but no model 
has been properly validated against all trends and all data on emission 
rates. 

The uncertainty of future projections may be examined by comparing 
different models or by varying parameters within a single model. Study 
of a group of models, each individually plausible, shows substantial 
agreement in projections for a fixed scenario of CO 2 input to the 
atmosphere. Maximum deviation of the lowest and highest concentration 
from the average among five models is less than 10%. Variation of 
parameters within plausible ranges in a single model shows at most 
about 30% variation from the mean (see Table 1.1). Thus, if current 
carbon-cycle models are accepted as valid representations of reality, 
reasonable variations in their parameters do not significantly affect 
predictions of future concentrations of C0 2 ; research simply to 
refine these parameters may not be effective in reducing uncertainties. 

On the other hand, if net releases of C0 2 from the biosphere 
comparable to those from fossil fuels are now in progress and have been 
for the past several decades, the question of carbon-cycle modeling is 
different. If, for example, C0 2 released annually from deforestation 
were in the upper part of the range that Woodwell suggests, the current 
models would fail to reproduce the observed atmospheric C0 2 growth 



TABLE 1.1 Sensitivity Study Using a Box Model of the Carbon Cycle 
(Keeling and Bacastow, 1977) and the Nordhaus-Yohe 50th Percentile 
C0 2 Emissions Estimate 

Ranged Ranged 
Variation in Parameter (ppmv) (%) 

Rate of exchange between air and sea 

2X and 0.5X standard rate of exchange 2 0.3 

Rate of exchange between mixed layer of 

the ocean and the deep ocean 

2X and 0.5X standard rate of exchange 70 9 

Both of above taken together 74 10 

Biospheric uptake due enhanced atmospheric CO^ 

No uptake and a 6 value of 0.266 229 29 

Buffer factor 

Constant (10) and variable according to 

predicted oceanic chemistry change 61 8 



The Range is the higher minus the lower predicted by the changes in 

arithmetic number used for the parameter in the year 2100. 

^Range divided by 784 ppmv, the predicted value for the year 2100, 

times 100. 

SThis is the C0 2 fertilization effect. 0.26 is the standard value 

for the so-called B -factor. 



21 

after 1958. The models would most likely have to be modified, since no 
reasonable adjustment of the parameters will allow a good fit of 
predictions to observations after 1958. The airborne fraction the 
ratio of atmospheric increase in a year to the net (fossil and biotic) 
amount added to the atmosphere would drop to about 0.3 from the value 
of almost 0.6 that is consistent with a net C0 2 release from the 
biosphere near zero. Increases of predicted concentration would be 
accordingly slower. 

While the sophistication of carbon-cycle models has been increasing, 
their predictive capability may diminish markedly as we depart from 
current CO 2 concentrations, reservoir sizes, and climate conditions. 
For example, the terrestrial biotic reservoir of carbon may increase or 
decrease in response to climate change as a result of warming, longer 
growing seasons, and change in rainfall patterns, for example. No 
model contains a satisfactory long-term treatment of climate feedbacks 
to the biosphere. For a decade, most carbon-cycle models in estimating 
biotic response have depended on the so-called Beta (?) factor, a 
measure of how much plant growth increases as a result of increase in 
atmospheric C02 concentration. The use of the $ factor needs to be 
replaced by separate analyses of effects of changes in the area of 
forests and potential changes in net ecosystem production caused by 
both increased atmospheric 002 and changes in climate. In sum, the 
current generation of carbon-cycle models appears to be satisfactory 
for forecasting concentrations in the next few decades, but credibility 
of the models fades as concentrations rise. 

Keeping in mind the state of the art of projecting CO 2 emissions 
and their partitioning among different reservoirs, we now report the 
estimates of Nordhaus and Yohe (this volume, Chapter 2, Section 2.1) 
and Machta (this volume. Chapter 3, Section 3.6) on possible future 
atmospheric concentrations and factors that affect them. Perhaps the 
most useful graph to study is Figure 1.10, which shows the percentiles 
of C02 concentrations for the different Nordhaus-Yohe emission 
trajectories. To calculate concentrations, Nordhaus and Yohe use an 
estimate of 0.47 as the fraction of current emissions that remains 
airborne during the first year after emission. (This amount is 
consistent with a historic average annual contribution of C02 from 
the biosphere of about 1 Gt of C.) For a quarter or half century, the 
inertia built into the world economy and carbon cycle leaves an 
impression of relative certainty about outcomes. After the early part 
of the next century, however, the degree of uncertainty becomes 
extremely large. The time at which CO 2 concentrations are assumed to 
pass 600 ppm, the conventional "doubling" of the concentration 
representative of the beginning of the twentieth century, can be shown 
as follows: 

Percentile Doubling Time 

5 After 2100 

25 2100 

50 2065 

75 2050 

95 2035 



22 



2500.CH 



1440 



2 1250.0- 



< 
<r 



LU 
o 

8 

O 
cc 

UJ 

I 



625.0 



312.5 




1975 



2000 



2025 2050 
YEAR 



2075 



2100 



FIGURE 1.10 Atmospheric concentration of carbon dioxide in parts per 
million. The indicated percentile runs for concentrations; the numbers 
on the right-hand side indicate concentrations in the year 2100 for 
each run. See Nordhaus and Yohe, Chapter 2, Section 2.1, and Figure 
2.18 for further detail. 



On this result, Nordhaus and Yohe base a central conclusion: Given 
current knowledge, odds are even whether the doubling of carbon dioxide 
will occur in the period 2050-2100 or outside that period. It is a 
l-in-4 possibility that doubling will occur before 2050 and a l-in-20 
possibility that doubling will occur before 2035. The median estimate 
for passing 600 ppm is 2065. For the year 2000, the most likely 
concentration is 370 ppm, with an upper limit of about 400 ppm. 

Nordhaus and Yohe also address the question of the relative impor- 
tance of different uncertainties in making concentration projections. 
Table 1.2 displays the contribution to overall uncertainty made by 
individual variables or parameters, calculated as the uncertainty 
induced when a parameter takes its full range of uncertainty and all 
other parameters are set equal to their most likely values. 



23 

TABLE 1.2 Indices of Sensitivity of Atmospheric Concentration in 2100 
to Uncertainty about Key Parameters^. (100 = Level of Effect of Most 
Important Parameter^) 

Marginal Variance 
from Most 
Likely Outcome 

Ease of substitution between fossil 

and nonfossil fuels 100 

General productivity growth 79 

Trends in real costs of producing energy 73 

Ease of substitution between energy and labor 70 

Airborne fraction for CC>2 emission 62 

Extraction costs for fossil fuels 56 

Population growth 36 

Fuel mix among fossil fuels 24 
Trends in relative costs of fossil 

and nonfossil fuels 21 

Total resources of fossil fuels 5 



For full explanation of parameters see Chapter 2, Section 2.1.2. 
^Value of sensitivity is scaled at 100 for the parameter that has the 
highest marginal variance. 

The ranking of the importance of uncertainties shown in Table 1.2 
contains several surprises. The most important parameters are those 
relating to future production trends , and the ease with which it is 
possible to substitute nonfossil sources of energy (e.g. r uranium) for 
fossil sources (e.g., coal) is at the top of the list; several of these 
parameters have rarely been noted as factors affecting future CO 2 
trends. Another surprise concerns two parameters that have been 
extensively discussed in the C0 2 literature: the extent of world 
resources of fossil fuels and the carbon cycle (based on a range for 
the airborne fraction of between 0.38-0.59).* The Nordhaus-Yohe 
estimates indicate that in projecting future CO 2 concentrations 
uncertainty about resource inventories is trivial. Uncertainty about 
the airborne fraction is of intermediate significance. 



*The airborne fraction for the last 25 years may have been less than 
0.38 if the higher figures reported by Woodwell in Chapter 3, Section 
3.3 f are accepted, and it could be greater than 0.59 in the future if 
the figures Woodwell reports are significant overestimates and the 
buffering capacity of the ocean declines; however, this range is a 
treatment of uncertainty roughly comparable with that used in 
Nordhaus-Yohe for the other parameters, which could also have 
values outside the range employed (see Chapter 2, Section 2.1). 



24 

Machta (this volume , Chapter 3, Section 3.6) employs a carbon-cycle 
model to consider C0 2 from deforestation as a possible real and 
important source of atmospheric C0 2 . If some reasonable amounts of 
future C0 2 from deforestation are added to C0 2 from future fossil 
fuel combustion, the error that would be introduced by the omission of 
the future deforestation C0 2 would be small in the year 2100 assum- 
ing/ say, a 2% per year growth rate in fossil fuel C0 2 after 1980. 
To give an extreme example, oxidizing 300 Gt of C,r\or about half the 
terrestrial biota, would result in an increase of^perhaps 75 ppmv in a 
predicted value of about 1,000 ppmv in the year 2100 or about 12% of 
the total increase. If the rate of emissions from fossil fuels is 
slower, then biotic emissions could account for a somewhat more sig- 
nificant share of overall increase. 

Nordhaus and Yohe have also made extremely tentative estimates of 
the effect of energy-sector policies designed to reduce the burning of 
fossil fuels, in particular the imposition of fossil fuel taxes, set 
for illustrative purposes at $10 per ton of coal equivalent. The taxes 
lower emissions noticeably during the period in which they are in place, 
but their effect on concentrations at the end of the twenty-first 
century is small. These examples suggest that use of carbon dioxide 
taxes (or their regulatory equivalents) will have to be very forceful 
to have a marked effect on carbon dioxide concentrations. 

A review of several studies (Chapter 2, Section 2.4), all quite 
tentative, shows that if fossil fuel growth rates are 1 or 2%/yr and 
concentrations of 400-450 ppm are judged acceptable, there is little 
urgency for reductions in C0 2 emissions below an uncontrolled path 
before A.D. 1990. The review suggests that if a limit in the vicinity 
of 450-500 ppm is desirable steps to reduce emissions below an uncon- 
trolled path would need to be initiated around A.D. 2000. 

Along with considering the climatic implications of increased CO 2 , 
it is important also to take into account other possible man-made 
changes in atmospheric composition (see Machta, this volume, Chapter 4, 
Section 4.3). Reliable measurements have now shown that background 
concentrations of several radiatively active gases besides C0 2 have 
increased worldwide in the 1970s. These include the chlorofluorocarbons 
CF 2 C1 2 and CPC1 3 , N 2 (nitrous oxide), CH 4 (methane), and 
ozone (in the troposphere) . Since these gases also absorb and emit 
thermal radiation, their effects on climate may add to those of C0 2 . 

Chlorofluorocarbons . This class of gases originates from industrial 
activities and has been emitted to the atmosphere during the past 50 
years. These gases are increasing in the atmosphere approximately as 
expected from their growth in emissions. CFC-11, CFC-12, and CFC-22, 
the three most abundant ones, all have long residence times in the air 
(tens of years) so that they can accumulate. Both the sources and sinks 
of the chlorofluorocarbons are believed to be known. The emissions 
from industrial production and products (such as aerosol propellants) 
represent the only source of any consequence. Photochemical destruc- 
tion, mainly in the stratosphere, and very slow uptake by the oceans 
are the only known significant sinks. Theoretically, chlorofluorocar- 
bons are implicated as potential destroyers of stratospheric ozone, the 
destruction of which in turn could result in damage to human health and 



25 

the environment from increased ultraviolet radiation. Since emissions 
of these gases may be increasingly restricted, an extrapolation of 
current or past growth rates of chlorofluorocarbons to predict future 
atmospheric concentrations may be misleading at this time. 

Nitrous oxide. It is likely that most nitrous oxide in the air has 
come from denitrification in the natural or cultivated biosphere. One 
would therefore expect to find the largest part of atmospheric nitrous 
oxide to be derived from nature , unrelated to human activity. Recent, 
careful measurements have suggested a small growth rate of the concen- 
tration of nitrous oxide in ground-level air at remote locations. The 
source of the small increase is unknown, but prime candidates are the 
continued expanded use of nitrogen fertilizers around the world to 
improve agricultural productivity and high temperature combustion in 
which atmospheric nitrogen is oxidized. If such fertilizers are the 
source, the current slow increase is likely to continue into the fore- 
seeable future because the demand for food will grow with population 
size. 

Methane. The most abundant hydrocarbon, methane, often called 
natural gas, is increasing in the atmosphere. It is thought to be a 
natural constituent of the air arising as it does from many biological 
processes and from seepage out of the Earth. Measurements in the 1950s 
and 1960s were imprecise, and there were spatial differences so that 
the observed temporal variability was not viewed as an upward trend. 
However, in the late 1970s several investigators using gas chro- 
ma tography have unequivocally demonstrated an upward trend. 

Increase in the number of ruminant farm animals and expansion of 
rice production might well explain, at least qualitatively, the atmo- 
spheric methane growth. Other biological activities, such as termite 
destruction of wood, and possible leakage from man's mining and use of 
fossil methane might also contribute to methane in the air, but their 
contribution to its increase is less clear. The higher concentrations 
far north of the equatorial region suggest that the termite source may 
be minor. The relatively rapid recent increase with time, about as 
fast as for C0 2 , combined with the uncertainty as to its origin, are 
both intriguing features of the methane growth in air. 

There is no reason to expect the upward trend in atmospheric methane 
concentration to stop soon since the most likely sources of methane are 
related to population size. In the long run, those sources that are 
dependent on the size of a biospheric feature (e.g., cows or rice 
paddies) will ultimately be limited by space. Thus, the growth in 
atmospheric concentration from these sources might continue for many 
decades but perhaps not for many centuries. 

Methane also forms another link in the question of future atmospheric 
composition. As discussed by Revelle (this volume, Chapter 3, Section 
3.5), large amounts of methane are believed to be stored in methane 
hydrates in continental slope sediments. Methane hydrate is a type of 
clathrate in which methane and smaller amounts of ethane and other 
higher hydrocarbons are trapped within a cage of water molecules in the 
form of ice. Methane hydrates are stable at low temperatures and 
relatively high pressures. With a rise in ocean-bottom temperatures, 
the uppermost layers of ocean sediments would also become warmer; and 



26 

methane hydrates would become unstable in the upper limit of their 
depth range, that is, about 300 m in the Arctic and about 600 m at low 
latitudes. The quantity of clathrates that will be released from 
sediments under the seaf loor as a result of ocean warming depends on 
the distribution of clathrates with depth and on their abundance in the 
sediments. Estimates of total amount by different authors differ by a 
factor of 500, from 10 3 to 5 x 10 5 Gt of C. Revelle, assuming a warming 
induced by 002 and other trace gases released by human activities and a 
stock of about 10 4 Gt of C, estimates that the resulting increase in 
atmospheric methane toward the latter part of the twenty-first century 
could be two thirds to four thirds of the current amount; the uncer- 
tainties in the seriej of methane calculations are so great that the 
result cannot be thought of as a projection for the future, but it is 
equally obvious that we must be attentive to the possibility of new, 
important feedbacks affecting the chemical composition of the 
atmosphere. 

Tropospheric ozone. Tropospheric ozone was originally believed to 
be primarily a consequence of transport from stratospheric ozone by air 
motions. It can also be created within the troposphere by man and 
nature. Locally, as in the Los Angeles Basin, large amounts of ozone 
are derived from reactions among oxides of nitrogen, hydrocarbons, and 
sunlight. Few scientists believe that these local sources of pollution 
can increase the upper- tropospheric concentrations of ozone, because 
ozone is so reactive that its lifetime in the lower atmosphere is no 
more than a few days. Nevertheless, an analysis of a limited number of 
measurements suggests an upward trend. It has been suggested that 
increase of mid- and upper-troposphere ozone concentration in the 
northern hemisphere may result from photochemical reactions of the 
oxides of nitrogen and hydrocarbons emitted by high-flying jet aircraft. 
Since the lifetime of an ozone molecule in the upper troposphere is 
also relatively short, little accumulation takes place. An increase in 
concentration must therefore reflect a continual increase in aircraft 
emissions, if they are the source. 

Some other gases. Several other gases being measured show upward 
trends and may have absorption lines in the infrared window of the 
electromagnetic spectrum, making them potential greenhouse gases, for 
example, carbon tetrachloride (CC 14 ) and methyl chloroform (CH 3 CC1 3 ) . 
Very likely both of these gases have both natural and man-made sources. 
On the other hand, measurements at the Mauna Loa Observatory exhibit no 
or insignificant increases in carbon monoxide (CO) . It is likely that 
the list of atmospheric gases studied for their trends and potential 
greenhouse effects will grow in years to come: the study of greenhouse 
gases other than C0 2 is still in its infancy. 

The atmospheric concentrations of these trace gases are not all inde- 
pendent of one another. Complicated chemical reactions among them, as 
well as with other gases not particularly radiatively active, can affect 
their concentrations. In addition to chemical reactions among today's 
atmospheric components, there are likely to be new climate-chemistry 
interactions in the future. As the atmospheric composition changes, 
the expected higher atmospheric water-vapor content will further affect 
the atmospheric chemistry. 



27 

Unlike C(>2, which generally does not undergo chemical changes in 
the air, these trace gases frequently do. Not only can the mean con- 
centration be affected by other chemicals and sunlight, but distribution 
particularly in the vertical can be influenced (ozone is a prime 
example) . To estimate future concentrations will require more than 
estimates of natural and man-made emission rates, fundamental though 
those rates will be. 



1-2.3 Changing Climate with Changing CO? 

With projections given of rising atmospheric concentration of C0 2 and 
other greenhouse gases, the next question to be addressed is that of 
possible effects on climate. The main tools for evaluating the 
possible role of greenhouse gases are numerical models of the climate 
system. Simplified models permit economically feasible analyses over a 
wide range of conditions, but they are limited in the information they 
can provide, for example, on regional climate change. More detailed 
inferences may be obtained primarily from three-dimensional general 
circulation models (GCMs) , which represent the global atmospheric 
circulation, as well as the oceans, the land, and ice (Figure 1.11) . 
Comparisons of simulated time means of a number of climatic variables 
with observations show that modern climate models provide a reasonably 
satisfactory simulation of the present large-scale global climate and 
its average seasonal changes. However, the capabilities of even the 
most advanced current models remain severely limited; for example, the 
three-dimensional GCMs are generally deficient in the treatment of 
ocean heat transport and dynamics and feedback between the ocean and 
the atmosphere. 

The adequacy and results of climate model studies in the CO 2 
context were examined in 1979 by an NRC panel chaired by the late Jule 
Charney, and again in connection with the present study by a panel led 
by Joseph Smagorinsky. Comprehensive reviews of modeling methods and 
results have been carried out by Schneider and Dickinson (1974) and 
international groups (Gates, 1979) . The Smagorinsky panel also 
evaluated empirical approaches to assessing climate sensitivity. The 
earlier NRC panel reports form one basis of our assessment of the 
climatic implications of increasing C0 2 . A second basis is the 
search for a "C0 2 signal" in the recent climatic record; this 
detection approach is discussed below in Section 1.2.4 and by Weller et 
al. in Chapter 5. 

The Carbon Dioxide Assessment Committee did not extend the 
systematic treatment of uncertainty adopted in several sections of this 
report to its analysis of climate modeling results. However, 
Smagorinsky (this volume, Chapter 4, Section 4.2) notes that such 
approaches are now being initiated by climate researchers. 

The primary effect of an increase of CO 2 is to cause more 
absorption and re-radiation of thermal radiation from and to the 
Earth's surface and thus to increase the air temperature in the lower 
troposphere. A strong positive feedback mechanism is the likely 
accompanying increase of moisture, which is an even more powerful 



28 



Changes of 
Solar Radiation 



SPACE 



ATMOSPHERE 



s, 3 , etc. 
dust particles 



ICE 




Air-ice Coupling 



T 



Precipitation, 
Evaporation 



Heat Exchange 



wind Stress 



Changes of 
Atmospheric Composition 



Ice-Ocean 
Coupling 





Atmosphere-Ocean Coupling OCEAN 



Changes of Land Features, 

Orography, Vegetation, 

Albedo, etc. 



EARTH 



Shape, Salinity, etc. 



FIGURE 1.11 Schematic illustration of the components of the coupled 
atmosphere-ocean-ice-earth climatic system. The solid arrows are 
examples of external processes, and the open arrows are examples of 
internal processes in climatic change. Source: U.S. Committee for the 
Global Atmospheric Research Program (1975) . 



absorber of terrestrial radiation. None of the known potential 
negative feedback mechanisms, such as increase in the areal extent of 
low or middle cloud amount, can be expected to vitiate the principal 
conclusion that there will be appreciable warming, since they do not 
appear in most current models to be as strong as the positive moisture 
feedback. There is, however, always the possibility of some overlooked 
or underestimated factor. 

When it is assumed that the C0 2 content of the atmosphere is 
doubled and statistical thermal equilibrium is achieved, all models 
2Sn* * u UrfaCe warmin 9- None of ^e calculations with more 
tions wS S mp " hens j ve models Predicts negligible warming. Calcula- 
tions with the three-dimensional, time-dependent models of the global 

f n CirCU i ati n indlCate gl bal warming due to a doubl i"9 of 
from 300 ppm to 600 ppm to be in the range between about 1.5 and 

' ** SUgge f te< l in 1979 by the Charne * P an *L Simpler models that 
to contain the main physical factors give similar results. 

K 2 lnferences suggesting negligible CO 2 -induced climate 

stupes to e bt S S ^^ ^ reC6nt YearS ' bUt Caref " l review ^ these 
studies to be based on incomplete and misleading analysis (National 



29 

Research Council, 1982; Luther and MacCracken, 1983). Investigations 
with a variety of climate models continue to show a broad range of 
estimates of appreciable warming* 

A major uncertainty has to do with the transfer of increased heat 
into the oceans, which serve as a thermal regulator for the planet. 
The heat capacity of the oceans is potentially great enough to slow 
down the response of climate to increasing C02 by several decades and 
to cause important regional differences in response. The influences of 
clouds, aerosols, and land-surface processes on sensitivity of climate 
to increased CO 2 also remain poorly understood. Table 1.3 indicates 
the possible contribution of various processes to the sensitivity of 
simple climate models. For example, including the feedback provided by 
reduction in the extent of snow and ice (Model 5) increases the 
equilibrium surface temperature rise for doubled C(>2 in the model by 
a factor of 1.3-1.4. 



TABLE 1.3 Equilibrium Surface Temperature Increase Due to Doubled 
CO 2 (300 ppm -> 600 ppm) in One-Dimensional Radiative-Convective 
Models* '' 



Model 


Description 


AT S (C) 


f 


F (W m~ 2 ) 


1 


FAH, 6. SLR, FCA 


1.2 


1 


4.0 


2 


FRH, 6.5LF, FCA 


1.9 


1.6 


3.9 


3 


Same as 2, except 










MALR replaces 6. SLR 


1.4 


0.7 


4.0 


4 


Same as 2, except 










FCT replaces FCA 


2.8 


1.4 


3.9 


5 


Same as 2, except 










SAF included^ 


2.5-2.8 


1.3-1.4 




6 


Same as 2, except 










VAF included^ 


3.5 


1.8 





^Source: National Research Council (1982) . 
^Data from Hansen et al. (1981) . 

^Model 1 has no feedbacks affecting the atmosphere's radiative 
properties. The feedback factor f specifies the impact of each added 
process on model sensitivity to doubled CO 2 . F is the equilibrium 
thermal flux into the planetary surface if the ocean temperature is 
held fixed (infinite heat capacity) when CO 2 is doubled; this is the 
flux after the atmosphere has adjusted to the radiative perturbation 
within the model constraints indicated but before the surface 
temperature has increased. ^ 

^FRH, fixed relative humidity; FAH, fixed absolute humidity; 6. SLR, 
6.5C km" 1 limiting lapse rate; MALR, moist adiabatic limiting lapse 
rate; FCA, fixed cloud altitude; FCT, fixed cloud temperature; SAF, 
snow-ice albedo feedback; VAF, vegetation albedo feedback. 
^Based on Wang and Stone (1980) . 
-Based on Cess (1978) . 



30 

Warming of the lower troposphere will be accompanied by regional 
shifts in the geographical distribution of the various climatic 
elements, such as temperature, rainfall, evaporation, and soil moisture. 
Indeed, variations with latitude, longitude, and season are likely in 
many cases to be more striking than globally averaged changes. Unfor- 
tunately, we cannot at present predict the magnitude and locations of 
regional climate changes with much precision or confidence. Although 
current models are not sufficiently realistic to provide reliable 
predictions in the detail desired, they do suggest scales and ranges of 
temporal and spatial variations. Along with global warming, the main 
conclusions of the model studies, discussed in greater detail in the 
report of the Smagorinsky panel (National Research Council, 1982) and 
in Chapter 4 of this report, may be summarized as follows: 

A cooling of the stratosphere with relatively small latitudinal 
variation is expected. 

Global-mean rates of both evaporation and precipitation are 
projected to increase. 

With less confidence it is concluded that: 

Increases in surface air temperature would vary significantly 
with latitude and over the seasons: 

(a) Warming at equilibrium would be 2-3 times as great over 
the polar regions as over the tropics; warming would probably be 
significantly greater over the Arctic than over the Antarctic. 

(b) Temperature increases would have large seasonal variations 
over the Arctic, with minimum warming in summer and maximum warming in 
winter. In lower latitudes (equatorward of 45 latitude) the warming 
has smaller seasonal variation. 

Some qualitative inferences on hydrological changes averaged 
around latitude circles may be drawn from model simulations of large, 
fixed CO 2 increase in equilibrium: 

(a) Annual-mean runoff increases over polar and surrounding 
regions. 

(b) Snowmelt arrives earlier and snowfall begins later. 

(c) Summer soil moisture decreases in large regions in middle 
and high latitudes of the northern hemisphere. 

(d) The coverage and thickness of sea ice over the Arctic and 
circum-Antarctic oceans decrease. 

As CO 2 slowly increases, land and ocean will both warm but at 
different rates. Eventually, with a steady CO 2 concentration, a new 
equilibrium would be reached with a climate different from today's. 
However, we should not expect that the detailed distribution and timing 
of changes during the intervening years can be obtained solely by inter- 
polation. The differences between rapidly heated land and slowly 
warming water, coupled with the irregular distribution of continents 
and oceans and continually changing CO 2 concentrations, may bring 
varying "transient 11 patterns of climate change. 



31 

While the climatic effects of C0 2 have been explored with a wide 
variety of climate models, most of the estimates of the climate effects 
of other greenhouse gases have been based on simple one-dimensional 
radiative-convective models. Typically the calculation involves 
doubling a reference concentration of the gas (for the chlorofluorocar- 
bons, increases from to 1 or 2 ppb are used) while other constituents 
are held constant. Table 1.4 gives some estimates of the change in 
surface temperature due to either a doubling of their concentration or 
an increase from to 1 ppb for the halocarbons. The table was adapted 
from Table 2a in the report of the World Meteorological Organization 
(1983). There are other published values, but they generally do not 
disagree by more than about +30% with the figures given here. 

The models used to obtain these results generally gave a sensitivity 
to doubled CO 2 between 2 and 3C. None of the changes of individual 
gases by itself approaches O> 2 , but it is clear that the summation of 
all of these potential changes could be of the same magnitude as C0 2 . 
It is worth noting that because the concentration of each of these 
gases is small enough for their radiative effect to be treated as 
optically thin, the temperature effect is linearly proportional to 



TABLE 1.4 Some Estimates of Surface Temperature Change Due to Changes 
in Atmospheric Constituents Other Than C0 2 



Mixing Ratio Change Si 
(PPb) T( 


irface 
emperature 
lange (C) Source 


Constituent 


From 


To Cl 


Nitrous oxide (N 2 0) 


300 


600 


0.3-0.4 


1/3 


Methane (CH 4 ) 


1500 


3000 


0.3 


3,4 


CFC-11 (CFC1 3 ) 





1 


0.15 


1/5 


CFC-12 (CF 2 C1 2 ) 





1 


0.13 


1/5 


CFC-22 (CF 2 HC1) 





1 


0.04 


7 


Carbon tetrachloride (CC1 4 ) 





1 


0.14 


1/5 


Carbon tetrafluoride (CF 4 ) 





1 


0.07 


2 


Methyl chloride (CH 3 C1 3 ) 





1 


0.013 


1/5 


Methylene chloride (CH 2 C1 2 ) 
Chloroform (CHC1 3 ) 






1 
1 


0.05 
0.1 


1/5 
1/5 


Methyl chloroform (CH 3 CC1 3 ) 





1 


0.02 


7 


Ethylene (C^) 


0.2 


0.4 


0.01 


1 


Sulfur dioxide (S0 2 ) 


2 


4 


0.02 


1 


Ammonia (NH 3 ) 


6 


12 


0.09 


1 


Tropospheric ozone (0 ) F(Lat f ht) 


2 F(Lat f ht) 


0.9 


4/6 


Stratospheric water vapor (H 2 0) 


3000 


6000 


0.6 


1 



^Sources: 1, Wang et al. (1976); 2, Wang et al. (1980); 3, Dormer 
and Ramanathan (1980); 4, Haraeed et al. (1980); 5, Ramanathan (1975); 
6, Fishman et al. (1979); 7, Hummel and Reck (1981). 



32 

their concentration r whereas the C02 effect depends logarithmically 
on the concentration. 

The implication of the prospective increases in trace gases is that 
climatic changes of the character expected for elevated C02 concen- 
trations may be encountered sooner than if CO 2 were the only cause of 
change, or r alternatively, that estimates solely of a C0 2 effect may 
be conservative. 

At present little or nothing can be estimated about changes in 
extreme conditions that might accompany changes in mean climatic 
conditions and more generally about the weather of future climate. We 
do not know, for example, whether the climate will show more or less 
year-to-year variability under generally warmer planetary conditions. 
Variability of climate is one of its most important features. Pood 
production, human settlements, and numerous aspects of the environment 
are strongly influenced by occasional extreme episodes. An area of 
particular interest is that of severe storms. The frequency, severity, 
and track of hurricanes and other severe storms are likely to be 
affected by C0 2 -induced climatic changes, such as warming of ocean 
waters. Neither our current knowledge of storm genesis nor the current 
capabilities of climate models are great enough to allow convincing 
linkages at this time. 

Besides numerical modeling approaches, past climates and recent 
climate fluctuations have been studied empirically to discern possible 
regional patterns of climatic variation associated with elevated C0 2 
levels or warmer mean temperatures. These studies can be useful in 
exploring the sensitivity of climate to various factors, in evaluating 
how well climate models perform, and in exploring the kinds of regional 
patterns of change that are possible. However, the search for a his- 
torical analogue to C0 2 -induced climatic change is hampered by 
inadequacies in data and by the absence of close parallels of cause and 
effect. Maps of a warmer earth derived from analogue approaches should 
not be viewed as predictions of regional effects of C0 2 -induced 
changes. 



1.2.4 Detection of CO^-Induced Changes 

Given the historic increase in atmospheric CO 2 and the results of 
climate models concerning the effects of increasing C0 2 , it is 
appropriate to ask whether climatic records tend to confirm model 
estimates. Weller et al. examine this question at length in Chapter 5 
of this report. Observational verification of model-based predictions 
is also important in calibrating climate models so that we can attach 
more confidence to their predictions of future changes. 

The most clearly defined change expected from increasing atmospheric 
C0 2 is a large-scale warming of the Earth's surface and lower atmo- 
sphere. Thus, a number of investigators have examined trends in 
globally or hemispherically averaged surface temperature for evidence 
of C0 2 -induced changes. Although differing in detail because of 
varying data sources and analysis methods, the records of large-scale 
average temperatures reconstructed by a number of investigators are in 



33 

general agreement for the period of instrumental records, i.e., about 
the last 100 years. Northern hemisphere temperatures, mostly measured 
over land in the 20-70 latitude zone, increased from the late nine- 
teenth century to the 1940s, decreased until the mid-1970s, and have 
apparently increased again in recent years (Figure 1.12). If one 
selects the 1970s to compare with the 1880s, one finds that the mean 
temperature of the recent decade was about 0.5C warmer; other selective 
examinations of the time series of northern hemisphere temperatures 
could show different results. To the extent that one can judge from 
scanty data, southern hemisphere temperatures have increased more 
steadily than in the north by about the same total amount. In view of 
the relatively large and inadequately explained fluctuations over the 
last century, we do not believe that the overall pattern of variations 
in hemispheric-mean or global-mean temperature or associated changes in 
other climatic variables either confirms or contradicts model projec- 
tions of temperature changes attributable to increasing atmospheric 
CO 2 concentration. 

Factors other than C0 2 such as atmospheric turbidity, solar 
radiation, and albedoalso influence climate. Attempts have been made 
to account for such influences on the temperature record and thereby 
make the sought-for C0 2 signal stand out more clearly. Unfortunately, 
only indirect sources of historical data are available. For example, 
stratospheric turbidity has been inferred primarily from volcanic 
activity, and solar radiance from phenomena such as sunspots. The 
quantitative reliability of these inferences is unknown. 



0.8 



0.4 



o 

o 

s 



-0.4 



-0.8 




U L. 



-I L. 



J L. 



JL 



J L 



1880 1895 1910 1925 1940 

YEAR 



-L 



Jones etal. (1982) 
Vinnikov et al. (1980) 

I 



1955 



1970 



1985 



FIGURE 1.12 Comparison of the reconstructions of annual surface air 
temperature anomalies for the northern hemisphere from Jones and Wigley 
(1980) and Vinnikov et al. (1980). Figure from Clark (1982), but data 
for 1981 added to Jones and Wigley (Jones, 1981). See Weller et al., 
Chapter 5, for further discussion. 



34 

Despite these diff iculties, a number of investigators, employing 
various combinations of data and methodology, have related the global 
or hemispheric mean temperature record to indices of turbidity and 
solar radiance and to estimates of the effect of increasing CO 2 . 
Although good agreement between modeled and observed variations has 
been obtained in some of these studies, it is clear that considerable 
uncertainties remain. When attempts are made to account for climatic 
influences of such other factors as volcanic and solar variations, an 
apparent temperature trend consistent with the trend in C(>2 concen- 
trations and simulations with climate models becomes more evident. 
However, uncertainties preclude acceptance of such analyses as more 
than suggestive. The studies done to date have been most helpful in 
raising questions, suggesting relationships, and identifying gaps in 
data and observations. 

In essence, the problem of detection is to determine the existence 
and magnitude of a hypothesized C0 2 effect against the background of 
quasi-random climatic variability, which may be in part due to internal 
processes in the atmosphere and ocean and in part explainable in terms 
of fluctuations in other external factors. A reasonable approach is to 
assume that the record of some climatic parameter, e.g., temperature, 
is the sum of a hypothesized "natural" value, a perturbation due to 
CC>2f and a random component. The "natural" value may be taken as a 
constant long-run preindustrial mean or perhaps that mean corrected for 
the factors discussed above. The random component will have statistical 
characteristics different from simple "white noise" and will be dif- 
ficult to model. It is clear that the magnitude of the derived C0 2 
signal will depend markedly on the hypothesis chosen for the unperturbed 
underlying climatic trend and the change in CO 2 assumed between the 
poorly known preindustrial value and the accurately measured current 
concentrations. The success achieved by several workers in explaining 
the temperature record in diverse ways demonstrates that a number of 
hypotheses can fit the poorly defined historical data and estimated 
preindustrial concentrations. 

The available data on trends in globally or hemispherically averaged 
temperatures over the last century, together with estimates of C0 2 
changes over the period, do not preclude the possibility that slow 
climatic changes due to increasing atmospheric C0 2 projections might 
already be under way. If the climate has warmed about 0.5C and the 
preindustrial C0 2 concentration was near 300 ppm, the sensitivity of 
climate to C0 2 (expressed as projected increase of equilibrium global 
temperature for a doubling of C0 2 concentration) might be as large as 
suggested by the upper half of the range indicated earlier, i.e., up to 
perhaps 4.5C; if the preindustrial C0 2 concentration was well below 
300 ppm and if other forcing factors did not intervene, however, the 
sensitivity must be below 3C if we are to avoid inconsistency with the 
available record (see Figure 1.13). 

If, as expected, the C0 2 signal gradually increases in the future, 
then the likelihood of perceiving it with an appropriate degree of 
statistical significance will increase. Given the inertia created by 
the ocean thermal capacity and the level of natural fluctuations, 
achieving statistical confirmation of the C0 2 -induced contribution to 



35 




230 240 250 260 270 



290 300 310 320 



[COJ 



2M850 



FIGURE 1.13 Relationship between C0 2 change/ temperature change f and 
climate sensitivity assuming no other factors intervene. The abscissa 
represents a range of values for the preindustrial (1850) concentration 
of CO 2- The ordinate represents the increase (AT) in global mean 
equilibrium surface temperature between 1850 and the period 1961-1980. 
The response is calculated for a range of values of AT^, the change 
of global mean equilibrium temperature for a doubling of CO 2 concen- 
tration (assumed independent of initial C0 2 concentration) 9 and 
assumes that the temperature range is logarithmically related to the 
change in C0 2 concentration (Augustsson and Ramanathan, 1977) . An 
ocean response time (mean thermal lag) of 15 years is used. The 
concentration of C0 2 was assumed in each case to increase linearly 
from the indicated value in 1850 to 310 ppm in 1950, and then linearly 
from 1950 to 340 ppm in 1980. Note that if the temperature increase 
from 1850 to the interval 1961-1980 is taken to be 0.5C, then for 
consistency, AT^ may be as large as 4.5C only if nineteenth 
century C0 2 concentrations were about 300 ppm, whereas AT d may be 
as small as about 1.5C if nineteenth century C0 2 concentrations were 
as low as 250 ppm. For ocean response times shorter than 15 years, the 
isolines slide upward and move in the opposite direction for longer 
ocean lag time. Varying the time of the start of the increase in C0 2 
concentrations from 1850 to 1920 has little effect. See Weller et al., 
Chapter 5, for further discussion. 



36 

global temperature changes so as to narrow substantially the range of 
acceptable model estimates may require an extended period. Improvements 
in climatic monitoring and modeling and in our historic data bases for 
changes in C0 2 , solar radiance, atmospheric turbidity, and other 
factors may, however, make it possible to account for climatic effects 
with less uncertainty and thus to detect a C0 2 signal at an earlier 
time and with greater confidence. A complicating factor of increasing 
importance will be the role of rising concentrations of greenhouse 
gases other than C0 2 . While the role of these gases in altering 
climate may have been negligible up to the present, their significance 
is likely to grow, and their effects may be indistinguishable from the 

effects of C0 2 . 

For purposes of analysis, in the next three sections we accept the 
estimates from models for C0 2 emissions, concentrations, and climate 
change and examine their implications for agriculture, water resources, 
and sea level and polar regions. For examination of agriculture, 
Waggoner looks ahead about 20 years and adopts maximum assumptions of 
change: about 400 ppm and a 1C warming. For examination of water 
resources, Revelle and Waggoner assume a 2C warming in mid latitude s; 
such a change could occur a few decades into the next century. For 
examination of sea- level change, a global average warming of about 
3-4C over about the next 100 years is assumed from a combination of 
C0 2 and other greenhouse gases. Our perspective is predominantly 
American. 



1.2.5 Agricultural Impacts 

Virtually all the food for the people of the United States and the feed 
for our animals grow on a third of a billion acres (1.35 x 10 12 m 2 ) 
of cropland and vast rangelands and pastures, exposed to the annual 
lottery of the weather and climate. Rather than seek to examine all 
aspects of agriculture that might be affected by C0 2 -induced climatic 
changes, we concentrate on a critical, susceptible, and illustrative 
aspect of agriculture: American crop production. These crops are 
critical to Americans because they feed us and bring $40 billion of our 
foreign exchange. They are also critical to others. For example, in 
1979 the United States provided 42% of the wheat and 19% of the rice 
traded between the nations of the world, and fully 43% of the world's 
corn crop is American. These crops may be susceptible and illustrative 
because most are grown in latitudes from 35 to 49, within a zone that 
climate researchers expect will experience a substantial change in 
weather and climate as C0 2 increases. Waggoner, in the detailed 
analysis in Chapter 6, concentrates particularly on wheat, corn, and 
soybeans, which outdistance in value any other American crop. 
From past experience we know that: 

Farmers fit husbandry and crops to weather and climate. 
environm ental change disrupts agriculture. 



37 

Pests can amplify effects of bad weather. 

The very soil can be changed by atmospheric conditions. 

Farm and range animals are affected by weather and climate. 

Occasional extremes destroy agriculture. 

Impact of changed weather is sharp in marginal climates. 

Against this background it is possible to calculate or speculate on 
the changes in crops that would follow hypothetical changes in the 
atmosphere. The hypothetical change that Waggoner considers is at the 
high end of the range estimated for the year 2000: an increase in 
C0 2 to about 400 ppmv, a mean warming of about 1C in the northern 
United States with a growing season about 10 days longer, and more 
frequent drought in the United States caused by somewhat less rain and 
slightly more evaporation. Regional studies in other areas or for 
other times might well begin by assuming different changes. 

Carbon dioxide is a major substrate for photosynthesis and, there- 
fore, can directly affect plant growth if a lack of CO 2 rather than a 
shortage of water or some other nutrient is the limiting factor. Since 
the current 340 ppmv appears to be limiting in many cases, a rise in 
atmospheric CO 2 should increase photosynthesis. However, most effects 
of C0 2 on photosynthesis and plant growth have been studied and 
measured during short periods when other factors such as light, water, 
temperature, and nutrients were adjusted to an optimal level. In 
addition, growth habits and adaptations to different environments might 
alter the effects of changing C0 2 concentration. 

Increased C0 2 affects photosynthetic rate and duration, as well as 
the fate and partitioning of photosynthate . Increased C0 2 may also 
improve the hydration of plants, because it influences the opening of 
the stomates, the pores through which plants gain C0 2 and transpire 
water. Increased production of photosynthates may improve the avail- 
ability of nutrients by encouraging growth of nitrogen-fixing 
symbionts, enlarging the pool of soil organic matter and increasing 
soil nitrogen levels. Changed environment not only affects crops but 
also weeds, pests, and their interrelationships, sometimes benefiting 
the crop, sometimes its competitors and predators. 

Although it is difficult to predict the changes in yield in real 
crops that might follow a rise in atmospheric C0 2 , a survey of 
experiments in growth chambers and greenhouses leads to the conclusion 
that C0 2 enrichment to 400 ppmv by A.D. 2000 may increase yields of 
well-tended crops by, say, 5% (see Table 1.5). In comparison, yield of 
Illinois corn has quadrupled in about half a century. Although the 
quantitative evidence for changes in yield under growing conditions in 
which factors other than C0 2 are limiting is equivocal, some increase 
in yield even in poor circumstances is indicated. 

Two orderly means are at hand to calculate how the yields of corn, 
soybeans, and wheat will change if rising C0 2 makes the climate warmer 
and drier. In one method, statistical regression, history is distilled 
to obtain the change in yield for a specified change in weather, such 
as the decrease in wheat yield in Kansas after a 10% decrease in March 
precipitation. In the other method, the physiology of, e.g., wheat and 
the physics of evaporation are assembled in a computer program or 



38 

TABLE 1.5 Changes in Yields of Crops in Optimum and Stressful 
Environments Anticipated from Atmospheric Enrichment to 400 ppmv of C0 2 





Change 


Yield 
Increment/ Yield 
C0 2 Change by 




in 


Increment 


Enrichment 








Yield 


Component 


(%/ppmv 


(%/60 ppmv 






Crop 


(%) Harvested of CO 2 ) 


of C0 2 ) 


Reference 




Optimum Environments 














Barley 


0.9i 


Grain 


0.18 


11 


Gifford et al. 


(1973) 


Corn 


0.28 


Young shoots 


0.03 


1.9 


Wong (1979) 




Cotton 


0.6 


Lint 


0.34 


20 


Mauney et al. 


(1978) 


Soybean 


0.41 


Grain 


0.04 


2 


Hardman and Brun 












(1971) 




Wheat 


0.4 


Grain 


0.13 


8 


Gifford (1979) 




Wheat 


0.3 


Grain 


0.07 


4 


Sionit et al. 


(1980) 


Wheat 


0.6 


Grain 


0.13 


8 


Sionit et al. 


(1981) 


Stressful Environments 


Corn 


0.28 


Young shoots 


0.03 


1.9 


Wong (1979) 




(1/3 normal N) 














Wheat 


0.6 


Grain 


0.44 


26 


Gifford (1979) 




(water limited) 














Wheat 


0.5 


Grain 


0.10 


6 


Sionit et al. 


(1980) 


(one H^ 














stress cycle) 














Wheat 


0.2 


Grain 


0.05 


3 


Sionit et al. 


(1980) 


(two H20 














stress cycles) 














Wheat 


0.1 


Grain 


0.02 


1 


Sionit et al. 


(1981) 


(1/8 normal 














nutrient) 















^Calculated from shoots only. 



"simulator"" of wheat to calculate the change in wheat yields per change 
in environment. Waggoner employs both these methods, comparing and 
verifying them as well as making predictions. Despite a long list of 
qualifications and warnings , a clear conclusion is obtained (see Table 
1.6). If we assume no significant adaptation of inputs and limited 
geographic mobility, the warmer and drier climate assumed to accompany 
the increased C0 2 will decrease yields of the three great American 
food crops over the entire grain belt by 5 to 10% f tempering any direct 
advantage of C0 2 enhancement of photosynthesis. 

Sometimes a change in the weather that has only a modest direct 
effect on a crop is amplified into a disaster by a third party, a pest. 
Although pests will change, we cannot predict how. 

Turning briefly to other countries and longer times, one can make 
some extrapolations. The direct benefit of more C0 2 to photosynthesis 
is universal and will continue for a long time. Crops in northern 
nations will benefit from warming, and tropical crops will be less 



39 
TABLE 1.6 Climate Change and Agricultural Productivity^ 

Estimated Change for 1C Temperature 
Increase and 10% Precipitation Decrease 

Crop and Present Yield Amount Percentage 

Region/State (quintals/hectare) (quintals/hectare) Change (%) 

Spring Wheat 

Red River Valley 18.2 -1.32 -7 

North Dakota 14.9 -1.77 -12 

South Dakota 12.0 -1.36 -11 



Winter Wheat 








Nebraska 


21.3 


-1.04 


-5 


Kansas 


21.3 


-1.04 


-5 


Oklahoma 


19.7 


-0.37 


-2 


Soybeans 








Iowa 


23.6 


-1.55 


-7 


Illinois 


21.9 


-0.82 


-4 


Indiana 


22.0 


-1.25 


-6 


Corn 








Iowa 


72.7 


-2.36 


-3 


Illinois 


68.8 


-1.72 


-3 


Indiana 


65.3 


-2.80 


-4 



^Examples of the effect of a hypothetical climate change on crop yields, if we 
assume no significant adaptation of inputs and limited geographic mobility. 
Results shown are based on statistical multiple regression analysis of observed 
crop and weather data and are calculated for a nominal 1C increase in 
temperature and 10% decrease in precipitation for each season or monthly period 
used as input to the analysis (see Chapter 6) . 



affected, if, as indicated by climate models, temperatures change 
little there. Where rainfall is now meager, an increase will have 
great benefit and a decrease can be tragic. Adaptation will be easier 
in countries that span several climatic zones and have sufficient 
wealth, ingenious farmers, and capable scientists with a practical 
outlook. Although adaptation will continue if the rise in CC>2 
continues for generations, ah accompanying and continuing desiccation 
could pass the ability to adapt. 

Returning to American crops, one sees in the end that the effects on 
plants of the changes in CC>2 and climate foreseen for A.D. 2000 are 
modest, some positive and some negative. The best forecast of yield 
for the next few decades in the United States, therefore, seems a 
continuation of the incremental increases in production accomplished in 
the past generation as scientists and farmers adapt crops and husbandry 
to an environment that is slowly changing with the usual annual 
fluctuations around the trend. 



40 
1.2.6 Water Supplies 

As discussed above in connection with agriculture/ C0 2 -induced climate 
changes would involve changes in precipitation/ temperature/ and their 
seasonal characteristics. Such changes must be expected to have conse- 
quences for rivers and thus for the availability of water for personal 
use, industry/ inland navigation/ and irrigation. Of the rain that 
falls on a given watershed/ a large part is eventually evaporated and 
transpired; the remaining runoff feeds the streams/ rivers, and aquifers 
that drain the region. Despite the current imprecision in predictions 
of climate changes/ it thus seems useful to consider their implications 
for runoff available for cities and irrigation. 

To assess the effects on the water resources of the United States of 
probable climate change, Revelle and Waggoner (Chapter 7) use the 
empirical relationships found by Langbein et al. (1949) among mean 
annual precipitation, temperature, and runoff. The catchments studied 
by Langbein and his colleagues were distributed over climates from warm 
to cold and from humid to arid/ but Revelle and Waggoner focus on the 
relations among runoff/ temperature/ and precipitation only for rela- 
tively arid areas. From Langbein 's data/ they observe that for any 
given annual precipitation/ runoff diminishes rapidly with increasing 
temperature. Similarly/ for any given temperature/ the proportion of 
runoff to precipitation increases rapidly with increasing precipitation. 

For any particular region, the relations derived are rather crude 
approximations because many physical factors, including geology, 
topography, size of drainage basin, and vegetation, may alter the 
effect of climate on runoff. Revelle and Waggoner believe, neverthe- 
less, that these relations can be used without serious error to 
describe the effects of relatively small changes in average temperature 
and precipitation on mean annual runoff. Table 1.7 shows the approxi- 
mate percentage decrease in runoff to be expected for a 2C increase in 
temperature. Table 1.8 shows the approximate percentage decreases in 
runoff for a 10% decrease in precipitation. From these exploratory 
investigations it is evident that in arid and semiarid lands relatively 
small changes in temperature or precipitation can produce amplified 
changes in runoff, river flow, and hence the availability of water for 
irrigation. 



1.2.7 Sea Level, Antarctic/ and Arctic 

Many processes can cause an apparent change in sea level at any 
particular location. They include local or regional uplift or 
subsidence of the land; changes of atmospheric pressure/ winds/ or 
ocean currents; changes in the volume of the ocean basins owing to 
volcanic activity, marine sediment deposition, isostatic adjustment of 
the Earth's crust under the sea, or changes in the rate of seafloor 
spreading; changes in the mass of ocean water brought about by melting 
or accumulation of ice in ice sheets and alpine glaciers; and thermal 
expansion or contraction of ocean waters when these become warmer or 
colder. Only the last two processes are of primary interest in con- 



41 

TABLE 1.7 Approximate Percentage Decrease in Runoff for a 2C Increase 
in Temperature^ 



Initial Temperature Precipitation (mm yr-1) 



200 



300 



400 



500 



600 



700 



- 2 


26 


20 


19 


17 


17 


14 





30 


23 


23 


19 


17 


16 


2 


39 


30 


24 


19 


17 


16 


4 


47 


35 


25 


20 


17 


16 


6 


100 


35 


30 


21 


17 


16 


8 




53 


31 


22 


20 


16 


10 




100 


34 


22 


22 


16 


12 






47 


32 


22 


19 


14 






100 


38 


23 


19 



^Source: Revelle and Waggoner, Chapter 7. Computed from data on 
runoff as a function of precipitation and temperature (Table 7.1) taken 
from Langbein et al. (1949) . 



sidering worldwide changes in sea level resulting from climate change, 
such as the warming that may be induced by increasing greenhouse gases 
in the atmosphere. (Melting or formation of sea ice and floating ice 
shelves have no effect on sea level a glass of ice water filled to the 
brim does not overflow while the ice melts.) But the other processes 
contribute to the "noise" that afflicts all sea-level records and may 
make their interpretation over periods of a few decades difficult or 
impossible. 

For orientation, it is useful to keep in mind that sea level has 
risen 150 m in the 150 centuries since the peak of the last glacial 
period. Hence, the present rate of 10-20 cm per century is small 
compared with the average rate of 1 m per century over the past 15 
millennia and very much smaller than the inferred maximum rise of 
perhaps 5 m per century immediately following the glacial period. 
Indeed, the present is a time of quiet sea level compared with the 
violent oscillations that occurred during most of the last 100,000 
years. 

The projected climatic warming from increasing atmospheric C0 2 and 
other greenhouse gases will lead to an increased transfer of water mass 
to the sea from continental (Greenland and Antarctic) and alpine 
glaciers. As shown by Revelle in Chapter 8, the resulting rise in sea 
level could be about 40 cm over the next century. Increased downward 
infrared radiation will also lead to a warming and, therefore, expansion 
of the upper ocean waters, which can contribute another 30 cm for a 
total of 70 cm. Assuming the correctness of the figure of 4 W m" 2 
for the increased downward infrared flux with a doubling of C0 2 



42 

TABLE 1.8 Approximate Percentage Decrease in Runoff for a 10% Decrease 
in Precipitation^, 



Temperature Initial Precipitation (mm yr-1) 

(C) 300 400 500 600 700 



- 2 


12 


16 


17 


18 


18 





14 


16 


17 


19 


19 


2 


15 


16 


19 


19 


20 


4 


17 


19 


19 


21 


21 


6 


23 


23 


21 


21 


21 


8 


30 


24 


24 


22 


22 


10 




24 


27 


23 


23 


12 




40 


30 


25 


25 


14 






34 


30 


27 


16 






50 


36 


29 



^Source: Revelle and Waggoner, Chapter 7. Computed from data on 
runoff as a function of precipitation and temperature (Table 7*1) 
taken from Langbein et al. (1949) 



(higher concentrations of other infrared absorbing gases might further 
increase this flux) , the estimates for both ice melting and ocean 
thermal expansion still have large uncertainty at least +25%, These 
are due to our uncertainty over the causes of the current rise in sea 
level, our inability to predict whether changes in atmospheric circu- 
lation will cause more or less snow to fall on the ice caps, our 
ignorance of the conditions for advance or retreat of alpine glaciers, 
and our lack of understanding of the physical processes associated with 
the flux of heat to the ocean. 

Of even greater uncertainty is the potential disintegration of the 
West Antarctic Ice Sheet, most of which now rests on bedrock below sea 
level. This could cause a further sea-level rise of 5 to 6 m in the 
next several hundred years. 

West of the Transantarctic Mountains (approximately from the 
Meridian of Greenwich, across the Antarctic Peninsula to 180 W) , most 
of the Antarctic Ice Sheet rests on bedrock below sea level, some of it 
more than 1000 m beneath the sea surf ace . In its present configuration, 
this "marine ice sheet 11 is believed to be inherently unstable; it may 
be subject to rapid shrinkage and disintegration under the impact of a 
C0 2 -induced climatic change (Mercer, 1978) . Events in the fairly 
recent geologic past suggest that rapid disappearance of West Antarctic 
ice has occurred before. However, evidence from radar soundings of 
flow lines extending across the Ross Ice Shelf indicates that the 
remaining West Antarctic Ice Sheet has been relatively stable for the 
last 1000-2000 years. Indeed, various lines of evidence suggest that 
the mass balance of the entire Antarctic Ice Sheet may be positive, 
i.e., ice may be accumulating. 



43 

If the West Antarctic Ice Sheet were to "collapse" (slide into the 
sea) , it would release about 2 million cubic kilometers of ice before 
the remaining half of the ice sheet began to float. 

The rate at which the West Antarctic Ice Sheet could disappear under 
the impact of a CC^-induced warming has recently been examined by 
Bentley (1983) . He concluded that rates of discharges and removal of 
icebergs might make disappearance barely possible/ although unlikely/ 
in 200 years, but only after removal of the ice shelves. 

If the time required for the ice shelves to disappear is 100 years, 
Bent ley's analysis would not be incompatible with a minimum time of 300 
years for disintegration of the West Antarctic Ice Sheet. The cor- 
responding average rate of rise of sea level would be slightly less 
than 2 m/100 years, beginning about the middle of the next century. 
Bentley's "preferred" minimum time of about 500 years would give a rate 
of sea-level rise of 1.1 m/100 years, which is, as pointed out earlier, 
about the mean rate for the last 15,000 years. To either of these 
figures we must add a rise of 70 + 18 cm between 1980 and 2080, which 
Revelle has shown is likely to result from ocean warming and ice 
ablation in Greenland and Antarctica, plus a possible retreat of alpine 
glaciers. These processes may well continue in later centuries. 

Like the Antarctic ice, Arctic ice has been a stable climate feature 
(see Annex 1). There is quite good evidence for persistence of the ice 
cover all year round for the last 700,000 years and perhaps for the 
past 3,000,000 years, although there is debate about whether the Arctic 
may have been open in summer from 700,000 to 3,000,000 years ago. The 
existence of glacial marine sediments in the Arctic basin shows that 
ice rafting occurred during the past 5,000,000 years. Longer ago than 
5,000,000-15,000,000 years, the Arctic may have been open year round. 
Global cooling patterns are such that an initial freeze-up of the 
Arctic may have occurred 15,000,000 years before the present, although 
there is no direct evidence. The physical reasons for the persistence 
of the Arctic ice are not well understood; but they may reflect both 
dynamic and thermodynamic processes, such that when little (excess) ice 
exists, correspondingly more (less) ice is produced the next winter. 

Studies on whether the Arctic sea ice will completely melt in 
summer, and if so, whether the ice will remain melted in winter, as 
suggested by Flohn (1983), have produced ambiguous results. Given the 
apparent long-term stability of Arctic ice, one must be cautious in 
projecting a melting due to prospective warming from increasing GO 2 
concentrations . A ju^er j^ that the 

5^ic_isj^ J5BY. JBSl.t.^la. summer ; with a _wari^Iag .o aboaJETthe magnitude 
that jnay^ be induced by_ a doubling of Cp2 and increase of other ^^ 
"gffeenhbuse gases, but this conclusion must be viewed as sOir tenta- 
-Tfve7The representations of the Arctic in energy balance and most 
climate models that have melted Arctic ice with a CO 2 warming usually 
do not include changes in cloud cover, ice dynamics, or the effects of 
open leads and salinity stratification. 

Owing to dynamic and thermodynamic processes, thickness of ice may 
respond more readily to temperature increases than extent of ice. 
However, verification of ice extent and thickness estimates from 
climate models is not yet adequate. 



44 

Oceanographic studies are also quite limited for the case of an open 
Arctic. There is now a very strong, salinity- induced, density 
stratification r the causes of which are not fully understood. If this 
stratification can be broken and does not reform, then the Arctic might 
be able to remain open through the winter. This possibility is not 
considered likely. 

Finally, there have been few studies of the effect of less ice or no 
ice in summer on atmospheric circulation. While atmospheric effects of 
reduction in Arctic ice remain highly speculative, some poleward shift 
of storm tracks seems likely and most significant climatic effects may 
occur during transition seasons. 



1.3 SERIOUSNESS OF PROJECTED CHANGES 

In assessing the seriousness of the changes projected in the preceding 
sections, there are two enormous sources of uncertainty. One source is 
the contents of the above outlook itself: uncertainties about sources 
and uses of energy, which in turn embody uncertainties about population, 
per capita income, energy-using and energy-producing technologies, 
density and geographic distribution of populations, and the distribu- 
tion of income; a multitude of uncertainties about the carbon cycle; 
uncertainties in translating a growth curve for O>2 in the atmosphere 
into appropriately time-phased changes in climate in all the regions of 
the globe; uncertainties about whether human activities other than 
release of C0 2 will be affecting the climate and what "natural" 
climatic trends will be; and, finally, uncertainties about effects on 
plant growth, water supplies, sea level, and other factors. 

The second source is uncertainty about the kind of world the human 
race will be inhabiting as the decades go by, through the coming 
century, and beyond. This source overlaps the uncertainties just 
mentioned; per capita income both influences the use of fossil fuels 
and affects how readily the world's population can afford, or can adapt 
to, changes in climate. And for both purposes the distribution of 
income the income disparities among different parts of the world, 
within countries as well as between countries affects the calculations. 
Similarly, the structures people inhabit, the ways people and goods are 
transported, the foods people eat, the ways countries defend themselves, 
and the geographical distributions of populations within and among 
countries, all affect land use and the kinds and amounts of energy used 
and hence the production of C0 2 ; but they also affect the ways that 
climate impinges on living and earning, even on what climates are 
preferred. The mobility of people, capital, and goods the readiness 
with which people can migrate, goods can be traded, and capital for 
infrastructure and productive capacity can flow among regions and 
countries would also determine how much difference the changes in 
climate would make. The location and significance of national 
boundaries and various international and supranational institutions 
would have much to do with whether adverse climatic effects in parts of 
the world could be offset, in a welfare assessment, by improvements in 
other places. Different individuals and groups will interpret these 



45 

uncertainties in different ways, depending on their culture, training, 
societal vantage point, and other factors. 

While emphasizing that the uncertainties just described must be kept 
in mind, we now discuss the areas of concern we have been able to 
identify in our discussions of the 002 issue. First we address areas 
of specifiable concern, then of more speculative concern, and, finally, 
of poorly defined but potentially serious concern. 



1.3.1 Specifiable Concerns 

1.3.1.1 Agriculture and Water Resources 

The outlook for American agriculture over the next couple of decades 
based on a foundation of physiology and history (summarized above and 
presented in detail by Waggoner in Chapter 6) tells how a C0 2 -induced 
change in climate would change the yields of crops if farmers, ignoring 
the weather, persisted in planting the same varieties of the same 
species in the same way in the same place. The safest prediction of 
any made by Waggoner is that farmers will adapt to a change in climate, 
exploiting it and, probably, proving our predictions to be pessimistic. 
If the climate changes, farmers will move themselves, change the crops, 
modify varieties, and alter husbandry. The loss of acreage to the 
margin of the desert, for example, may be replaced by yield and acres 
at the cold margin. Seeking higher yields and more profit, farmers 
will correct their course annually, and they may even adapt to a slowly 
changing climate unconsciously and successfully. Thus, we do not regard 
the hypothesized C0 2 - induced climate changes as a major direct threat 
to American agriculture over the next few decades. Of course, shifts 
at the margins may be easy for the nation but not for those involved. 

While the effects of increased C02 plus climatic warming on 
agriculture might be relatively small in the United States, such effects 
might be much larger in countries that do not have or build a good 
agricultural research infrastructure or the agricultural flexibility 
that come from relatively large capital investments in agriculture, 
rural transportation, farm credit and crop insurance, and marketing 
systems. Through trade linkages and political and social awareness, 
climate-induced agricultural problems in other parts of the world will 
become America's concern. Longer-term agricultural impacts, as global 
climatic conditions continue to shift, perhaps at an increasing rate, 
might also conceivably be much more serious. Will these be offset by 
the benefits of C0 2 for photosynthesis and water-use efficiency? At 
present we lack tools with which to evaluate in a credible way the very 
long-run prospects. 

While on balance U.S. agriculture as a whole may not suffer signifi- 
cantly, irrigated agriculture is both important and susceptible. Its 
importance derives from its expanse and the value of its crops. Fully 
50 million acres, or about 1 in 7 American acres, of cropland are 
irrigated. The quarter trillion cubic meters of irrigation water 
withdrawn from American streams and groundwater represent about half of 
all withdrawals of this natural resource. Averaging wheat yields over 



46 

all American fields, humid as well as arid, one sees an average of 1.9 
tons/ha on unirrigated versus 3,7 tons/ha on irrigated land. Valuable 
crops are grown on irrigated fields because irrigation reduces vari- 
ability of water and produces consistently high yields. Thus, most of 
the irrigated cropland (44 million acres) occurs on only 12% of the 
farms, but these farms produce fully 40% of the market value of the 
crops from all American cropland ( (Jensen, 1982) . 

Irrigation is susceptible, because it is such a heavy user of water. 
In seven U.S. water regions examined by Revelle and Waggoner (Chapter 
7) , the share of total water withdrawals for irrigation ranged from 68% 
to 95%. For these regions, Revelle and Waggoner draw on the efforts of 
Stockton and Boggess (1979) and perform a corroborating study of the 
Colorado River to estimate the effects of a climatic change on water. 

Revelle and Waggoner (Chapter 7) show that the effect of climatic 
change on water must be considered separately for different regions. 
Following Stockton and Boggess (1979) , they make several simplifying 
assumptions, the most important of which are: 1) variations in annual 
runoff are predominantly influenced by climate, although other factors, 
such as geology, topography, and vegetation, have effects, and 2) that 
evapotranspiration is controlled only by temperature. A warmer and 
drier climate would severely affect the seven water regions: the 
Missouri, Texas Gulf, Rio Grande, Arkansas-White-Red, Upper Colorado, 
Lower Colorado, and California. All are in the western United States; 
and although they cover about half the country, they have less than 15% 
of the runoff. A 10% decrease in precipitation, combined with a 2C 
warming, would decrease runoff in these regions between 40 and 76% (see 
Table 1.9). The impact would be especially severe in the Missouri, Rio 
Grande, Upper Colorado, and Lower Colorado regions where even current 
water requirements would exceed the supplies after climatic change by 
between 20 and 270%. Local shortages and a general deterioration of 
water quality would occur in the Arkansas-White-Red, Texas Gulf, and 
California regions. Much of the irrigated area might have to be 
abandoned unless water could be imported from other regions with more 
abundant supplies, such as the Pacific Northwest or the Upper and Lower 
Mississippi. Major additions to reservoirs would be required in several 
regions to maintain a safe yield of water during drought, even after a 
reduction of irrigated area and the maximum practicable rise in the 
efficiency of the use of water in irrigation. 

Except for the Rio Grande, the Colorado River is more intensively 
used than any other major stream in the United States. Half the 
estimated "normal" river flow of 18 billion cubic meters per year has 
been allocated by interstate compact, confirmed by federal law, to the 
"lower basin" states of Arizona and California, with minor amounts 
going to Nevada, although nearly all of the runoff originates from snow 
in the high mountains of western Colorado, southwestern Wyoming, and 
eastern Utah. Revelle and Waggoner examined mean annual precipitation, 
temperature, and river flow for 1931 to 1976 and found a high correla- 
tion between variations in precipitation and temperature, on the one 
hand, and runoff on the other. A rise of 2C in average temperature 
from 4.2 to 6.2C would reduce runoff by 29 + 6%, and a 10% decrease in 
precipitation would cause a further reduction of about 11 + 1.4% in 



47 







c 
o >1 








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TABLE 1.9 C 
Increase in 




Water Region^ 


i) o O 

4J -D 'O 

H id (d 

jC MM 
S H 0) <d 
1 H -o H H -n 
H CO 3 C O C 

S g 2 83S 

C CO C2) M M MH 

ca of <d cu <u -H 

CO * X Oi H 
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S < fri tf D J 



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the 7 regions 
ether 



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'H o <U 6 
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C QJ ^ -H to 
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<d rH O 



c 


4J 




C (D D) CO 03 CO 
-H 4J C 0) <D 0) -H 
0) W 



-H D> T3 'O 





48 

runoff. A rise in temperature even without a decrease in 
precipitation would seriously affect entire states. 

A 2C warming and a 10% reduction in precipitation would probably 
not have serious effects on water supplies in the humid regions east of 
the 100th meridian. Neither would effects be severe in the water-rich 
Pacific Northwest and the Great Basin (parts of Nevada, Utah, and 
Idaho) , where demand is relatively small and groundwater reserves are 
large. 

Loss in irrigated yield accompanying a change in climate can be 
envisaged in two ways. For grain, one might simply and roughly say that 
some areas can no longer be irrigated, and on those acres the yield will 
be reduced by at least 50% because the subsequent dryland crops will be 
grown in alternate years. For produce, a decrease in water and irri- 
gated area for truck croplands could reduce yields to zero on many acres 
and thus decrease the supply of fresh vegetables in the supermarkets, 
especially in the winter. Of course, decline of irrigation systems, 
often caused by increasing salinity and rising water table when drainage 
facilities are inadequate, is not a new experience for mankind. 



1.3.1.2 Rising Sea Level 

If one accepts the projections of warming, then certain physical 
consequences seem inescapable. One of these is the slow rise in global 
sea level. As explained by Revelle, melting of land ice and thermal 
expansion of the ocean may lead to, a rise of about 70 cm in global sea 
level over the next 100 years, continuing thereafter. Many shoreline 
problems (for example, coastal erosion, storm surges, and salinity of 
groundwater) are sensitive to sea-level changes on the order of deci- 
meters, and 70 cm, though modest-sounding on a calm day at the sea- 
shore, could effect a variety of unwelcome changes. We discuss the 
question of larger and continuing sea-level rise in the section that 
follows . 



1.3.2 More Speculative Concerns 

Our more speculative concerns center on the West Antarctic Ice Sheet 
(WAIS) and sea-level rise, the Arctic, and human health. These concerns 
are more speculative in that both the scientific uncertainties are 
greater and the potential effects are more distant. 

Resuming the question of sea level, we concluded above that with a 
postulated warming of about 3 or 4C from C0 2 and other greenhouse 
gases a gradual rise is probable over the next 100 years as a result of 
thermal expansion of the ocean, ablation of the Greenland and Antarctic 
ice caps, and retreat of alpine glaciers. We have also mentioned that, 
because of events in Antarctica, a much larger rate of rise is not 
unlikely during the following several centuries. Rates of sea-level 
rise could reach 1-2 m per 100 years. A complete collapse of the WAIS 
would produce a worldwide rise in sea level of between 5 and 6 m. 



49 

How serious would such rates and levels of sea-level rise be? As 
Schelling discusses in Chapter 9, there are three principal ways that 
human populations can adapt to a rising sea level: retreat and 
abandonment, construction of dams and dikes, or building on piers and 
landfill. The basic division is between abandonment and defense. 

Defense against sea-level rise has received little attention in this 
country. It is therefore worth emphasizing that there are ways to 
defend against rising sea levels. For built-up and densely populated 
areas, defenses could be cost effective for a rise of as much as 5 or 6 
m. Even where defending against 5 m would not be cost effective, 
defending against a meter or two could make sense for a century or 
two. Defense is not an empty hypothetical or purely speculative option. 

The economics of dikes and levees depends on the availability of 
materials (sand, clay, rock); on the configuration of the area to be 
protected; on the differential elevation of sea level and internal 
water table; on the depth of the dike where it encloses a harbor or 
estuary; on the tide, currents, storm surges, and wave action that it 
must withstand; and on the level of security demanded for contingencies 
such as extreme ocean storms, extreme internal flooding, earthquakes, 
military action, sabotage, and uncertainties in the construction itself. 
On the economics of diking, it is worth remembering that the Dutch for 
centuries have found it economical to reclaim the bottom of the sea, at 
depths of several meters, for agricultural, industrial, and residential 
purposes. 

The situation is totally different for an area like the coast of 
Bangladesh. Defense would be extremely costly for a region with a huge 
coastal area subject to inundation, rather than a concentration of 
capital assets that could be enclosed by a few miles of dikes. Such an 
area would be so susceptible to internal flooding with freshwater that 
levees required to protect the country would be many times greater than 
the length of the shoreline. 

Where defense is not practicable, retreat is inevitable, at least 
selectively. In urban concentrations, where buildings may last a 
century, good 100-year predictions of sea-level change (including 
likely erosion and storm damage) might permit the orderly evacuation 
and demolition of buildings without excessive write-off of undepreciated 
assets. 

Changes in Arctic weather and climate would have both practical and 
noneconomic implications. Open seas and easier ice conditions would 
have bearing on long-term strategies of use for northern seas and 
channels with respect to both navigation and seafloor development. Oil 
and gas exploration, drilling, production, and transportation could 
become easier and less expensive. The old dream of a "Northwest 
Passage" might become a reality. An ice-free Arctic Ocean in summer 
and a less hostile environment in North American, Russian, and 
Scandinavian Arctic regions would also have implications for military 
strategy and tactics, if technology does not shift military issues to 
other spheres entirely. For example, surface and aircraft-carrying 
fleets could operate in the Arctic during the summer months , as they do 
now in other oceans. One effect of warming should be a change in the 
stability and distribution of permafrost. This change would, in turn. 



50 

suggest design changes for overland vehicles f construction equipment, 
pipelines, and buildings. On a different plane, concern arises about 
possible loss of habitats and the conservation of nature; polar regions 
are among the wilder and more pristine environments remaining. 

In contrast to polar and sea-level change, not much consideration 
has been given by those who study increasing C0 2 and climate change 
to any possible direct effect on human health or the animal population 
from C0 2 in the air we breathe. The natural a priori concern with 
the health effects of a doubling or quadrupling of an important gas in 
the air we breathethe substance that actually regulates our breathing 
rate is relieved by the observation that for as long as people have 
been living indoors, not to mention burning fuel to heat themselves, 
they have been spending large parts of their lives virtually entire 
lives in the case of people who work indoors and travel in enclosed 
vehicles in an atmosphere of elevated C0 2 . Doubling or even quad- 
rupling C0 2 would still present a school child with a lesser concen- 
tration during outdoor recess than the child faces in today's average 
classroom* 

There is, furthermore, no documented evidence that C0 2 concen- 
trations of five or ten times the normal outdoor concentration damage 
human or animal tissue, affect metabolism, or interfere with the 
nervous system. Nor is there a theoretical basis for expecting direct 
effects on health from the kinds of C0 2 concentrations anticipated. 

But even though this answer is reassuring, the question has to be 
faced. It will occur to people who hear about changes in the atmosphere 
that their grandchildren are going to breathe. And experiments have 
not been carried out with either people or large animals whose whole 
lives, including prenatal life, were spent in an environment that never 
contained less than, say, 700 ppmv of C0 2 . So the question deserves 
attention, even though there is no known cause for alarm. 

Probably more serious is the effect of elevated temperatures on 
health and welfare. If a 3 or 4<>C increase in average temperatures 
C ^ rS ' S * ight . be ^cted in different parts of the United States 
rise bv an SK" 9 ' *? "f SUmmer tem P eratu s in warm years might 
rise by an equal amount. Excess human death and illness are already 

S! a ^ te u iStiC f SUmmer " hot s P ell ^ B and these might be worsened by 
much higher extreme summer temperatures. And, climatic shifts may 
change the habitats of disease vectors or the hosts for such vectors. 

1.3.3 The Problem of Unease about Changes of This Magnitude 
3^^.Tl f 2 ^ "" SPeCUlati - erns about impacts of 

==r-2s 

other greenhouse gases? D increases in atmospheric C0 2 and 



uon f * % x -Ple, the most frequently 

measure of climate teSsufSt^T SUrfaCe tem Pature. This crude 

- 



51 

would be experienced* Global average surface temperature has come to 
such prominence in large part because it represents a relative measure 
of 002 effects among climate models. Indeed, for many models it is 
the only result with much scientific validity. Nevertheless, changes 
in average surface temperature may suggest well the nature of our 
unease . 

Increasing CO 2 is expected to produce changes in global mean 
temperature that, in both magnitude and rate of change, have few or no 
precedents in the Earth's recent history. Consider the ranges of 
temperature experienced in various periods in the past (Figure 1.14) . 
A range of less than a degree was experienced in the last century, less 
than 2C in the last thousand years, and only 6 or 7C in the last 
million years. The development of civilization since the retreat of 
the last glaciation has taken place in a global climate never more than 
1C warmer or colder than today's. Despite the modest decline of 
time-averaged global-mean temperatures since the 1940s, we are still in 
an unusually warm period in the Earth's history. Indeed, according to 
one source (Jones, 1981) , 1981 was the warmest year on record. Thus, 
the temperature increases of a couple of degrees or so projected for 
the next century are not only large in historical terms but also carry 
our planet into largely unknown territory. Increasing CO2 promises 
to impose a warming of unusual magnitude on a global climate that is 
already unusually warm. 

Furthermore, the question of threshold responses arises. It is 
possible that a change in the central tendency of climate will come 
about smoothly and gradually. It is also possible that discontinuities 
will occur. For example, Lorenz (1968) and others have suggested the 
possibility of more than one climatic equilibrium. 

As Schelling (Chapter 9) points out, our calm assessment of the C0 2 
issue rests essentially on the "foreseeable" consequences of climatic 
change. Less well-seen aspects remain troubling. We have mentioned 
the possible release of methane clathrates from ocean sediments. We 
have also mentioned melting of the central Arctic sea ice. Disappear- 
ance of the permanent Arctic ice would result in a marked increase in 
the thermal asymmetry of the planet, with only one pole still glaciated. 
Such asymmetric conditions could produce further, unanticipated 
climatic changes (Flohn, 1982) . Warming amplified at high-latitude 
regions could also affect major features of the oceanic circulation, 
and these too could lead to unexpectedly different climatic conditions, 
as well as changes in the capacity of the oceans to absorb C0 2 . At 
the level of ecosystems, surprising changes may also result from 
climatic shifts. 

We are not complacent about global-average temperature changes that 
sound small; very serious shifts in the environment could well be 
implied. There is probably some positive association between what we 
can predict and what we can accommodate. To predict requires some 
understanding, and that same understanding may help us to overcome the 
problem. What we have not predicted, what we have overlooked, may be 
what we least understand. And when it finally forces itself on our 
attention, it may appear harder to adapt to, precisely because it is 
not familiar and well understood. There may yet be surprises. Antici- 



52 




1.0 0.8 0.6 

10 5 YEARS AGO 









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53 

pating climate change is a new art. In our calm assessment we may be 
overlooking things that should alarm us. 

At the same time, one might observe that barring the kind of 
surprises mentioned above the climate changes under consideration are 
not large in comparison with the climate changes individuals and social 
groups have undergone historically as a result of migration. Table 
1.10 shows U.S. population for 1800, 1860, 1920, and 1980, distributed 
according to the climatic zones in Figure 1.15. These data have been 
transformed into a series of maps of the United States in which the 
areas of our various climatic zones are drawn so as to be proportionate 
to their populations at various times (see Chapter 9) . The maps 
seemingly depict massive climate change; formerly empty, thus small, 
climatic zones become heavily populated and grow large. But it is not 
that deserts have expanded or that the climate has changed from perma- 
frost to rain forest, or from prairie to Mediterranean west coast, or 
to places where it gets cold but does not quite freeze from where it 
got a little colder and did freeze. People have moved, and to all 
climates, to places of enormous extremes like the Dakotas and places of 
little change like Puerto Rico. People have moved from the seacoast to 
the prairie, from the snows to the Sun Belt. 

Not only have people moved, but they have taken with them their 
horses, dogs, children, technologies, crops, livestock, and hobbies. 
It is extraordinary how adaptable people can be in moving to drastically 



TABLE 1.10 U.S. Population by Climatic 



Climatic 
Zone2 


Description 


Population 








1800 


1860 


1920 


1980 


Aw 


Tropical wet and dry 





2,996 


129,741 


2,793,140 




(Savannah) 




( 1) 


( 1) 


(1) 


BS and BS k 


Semiarid and steppe 





64,018 


4,291,664 


21,000,465 








( 1) 


(4) 


(9) 


BW h 


Tropical and subtropical 





28,029 


743,263 


4,955,742 




desert 




( 1) 


( 1) 


(2) 


Caf 


Humid subtropical 


2,034 f 536 


9,426,517 


32,360,561 


71,932,014 




(warm summer) 


(42) 


(32) 


(29) 


(32) 


Cb 


Marine (cool summer) 





39,246 


1,795,406 


4,447,811 








( 1) 


(2) 


(2) 


Cs 


Dry-summer subtropical 





202,420 


1,636,597 


8,675,763 




(Mediterranean) 




( 1) 


(2) 


(4) 


Daf 


Humid continental 


2,348,030 


16,074,866 


59,811,474 


90,882,262 




(warm summer) 


(49) 


(54) 


(54) 


(40) 


Dbf 


Humid continental 


435,665 


3,586,555 


9,394,792 


13,710,636 




(cool summer) 


(9) 


(12) 


(8) 


(6) 


H 


Undifferentiated 





184,896 


1,559,963 


9;147,733 




highlands 




( 1) 


(1) 


(4) 



^Source: U.S. Census Bureau, 1800, 1860, 1920, 1980. Data compiled by Clark 
University Cartographic Service. 

Figures in parentheses are percentage of total population in that climate zone. 
2Climatic zones shown in Figure 1.15. 



54 




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55 

different climates. That adaptability may suggest that if climates 
change only by shifting familiar climates around the world, it is not 
altogether different from leaving the climates alone and moving the 
people around. Of course, when people moved from England to Massa- 
chusetts or from the East Coast to the Great Plains, there were 
substantial difficulties in adapting; and if the climate changes and 
people stay, they may also have substantial difficulties. But it 
appears that a change in the climates where people live may not be 
altogether different from people moving to another climate. It may be 
that what we have to look forward to is not quite so historically 
unusual as a human experience as the descriptions from the paleoclimatic 
record would suggest. We have really become accustomed to marked 
climate change. For the individual, in contrast to the environment, 
the idea of climate change in a generation or two is far from novel. 

While people may be able to adapt readily to climatic change, they 
may be unwilling to accept climatic changes imposed on them involun- 
tarily by the decisions of others. Thus, in trying to clarify our 
unease about CC>2- induced climatic change, it is necessary to point 
out the potentially divisive nature of the issue. It is important to 
recognize the distribution of incentives for, and effects of, human- 
induced climatic changes. Although it might be in the interest of the 
world economy to restrict, at some cost, the use of fossil fuels, it is 
probably not in the interest of any single region or nation to incur on 
its own the cost of reduction in global C0 2 . For example, countries 
that view heavy rains as disasters and countries that view them as 
water for their crops would have different preferences about which, if 
any, rains to avoid or restore and whether they or another country 
should forgo (or burn) fossil fuels to help effect the change. The 
marginal effects of climatic change on the distribution of wealth may 
range from quite positive to quite negative. In short, CC>2-induced 
climatic changes, and more generally weather and climate modification, 
may be a potent source of international conflict. 



1.4 POSSIBLE RESPONSES 

So far we have developed an outlook for CO 2 -induced climate change 
and made some tentative evaluations of the seriousness of possible 
changes in prospect. In the preceding discussions we have occasionally 
referred to potential societal responses, for example, taxes on C0 2 
emissions, agricultural adjustments, and migration. Now we discuss 
possible responses in a more systematic fashion and offer two sets of 
comments. One set relates to flexibility in defining the issue, the 
other to specific categories of response. 



1-4.1 Defining the Problem 

As Schelling points out in Chapter 9, how one defines a problem or 
issue often governs or biases the search for solutions and sometimes in 
a way that puts emphasis on more difficult or less attractive solutions. 



56 

The protagonist of this study has been CO 2 . Recent research reflected 
in this report has been largely motivated , first/ by the observation 
that atmospheric C0 2 is increasing as the use of fossil fuels expands 
and/ second, by the known potential for a "greenhouse effect" that could 
generate quantitatively significant changes in climate worldwide. The 
members of the group responsible for the report are known as the Carbon 
Dioxide Assessment Committee, the work was authorized by an Act of 
Congress concerned with carbon-intensive fossil fuels, and the agency 
principally charged with managing the research is the Department of 
Energy. The topic is generally referred to as "the carbon dioxide 
problem," a global challenge to the management of energy resources. 

However, there are good reasons for taking climate change itself as 
the main perspective rather than CO 2 or energy. One reason is that 
over the span of time that this report has to cover there could be 
changes and fluctuations in climate not due to human activity. A second 
reason is that C02 is not the only climate-affecting substance that 
society releases to the atmosphere. Not only must the impact of C0 2 
be assessed in conjunction with other climate-changing activities, but 
any policy response needs a focus broader than CO 2 . A third reason 
is that there is a natural tendency to define a problem by reference to 
the agent of change and to seek solutions in the domain suggested by 
the naming of the problem, e.g., "fossil fuels 11 or "C02- n 

There is a legitimate presumption that where the Earth's biosphere 
is concerned any drastic change may produce mischief. There is also a 
widespread methodological preference for preventive over alleviating 
programs, and for dealing with causes rather than symptoms. But it may 
be wrong to commit ourselves to the principle that, if fossil fuels and 
C0 2 are where the problem lies, they must also be where the solution 
lies. Although a precautionary attitude toward any drastic changes in 
world climates would be prudent, definition of the problem requires 
investigation not only of what changes in climates may occur but also 
of what damages or blessings the changes may bring. 

To illustrate the point, while for any expected adverse consequences 
of C0 2 , conserving fossil fuel is an obvious policy option at the 
outset, the parallel importance of water supply and conservation emerges 
only later. Defining the issue as "the C0 2 problem" can focus atten- 
tion too exclusively on energy and fossil fuels and divert it from 
rainfall or irrigation or, more even-handedly, the broad issue of 
climate change. 

Something else is illustrated about the character of the issue. If 
the solution has to be reduced C0 2 emissions, both the problem and 
the solution are global in a severe sense. A ton of C0 2 produced 
anywhere in the world has the same effects, for good or ill, as a ton 
produced anywhere else. Any nation or locality that attempts to 
mitigate prospective changes in climate through a unilateral program of 
conservation, fuel switching, biomass enhancement, or scrubbing of 
C0 2 from smokestacks, in the absence of some global fuel rationing or 
compensation arrangement, pays alone the cost of its program while shar- 
ing the consequences with the rest of the world. In contrast, water 
resources are usually regional or local. Worldwide agreements involving 
some of the main consumers or producers of fossil fuels would be 



57 



essential to programs for reducing CO 2 emissions? in contrast, water 
development and conservation are national in scope or involve a few 
neighboring countries. 



1.4.2 The Organizing Framework 

If we accept that the issue is climate f then it follows that the 
organizing framework for welfare and policy implications of atmospheric 
C0 2 should also be built around climate change, not around CO 2 . 

As Schelling argues, the framework ought to be comprehensive. It 
should include theoretical possibilities that may be of no contemporary 
significance, because we have to think about choices as they evolve 
through the next century. The framework should make room for imagina- 
tion, not just for options that currently look cost effective. 

The framework should lend itself to different levels of universality. 
While atmospheric C0 2 is a global condition, its consequences and 
many of its policy implications will be regional and local. Governments 
will assess consequences and choose policies according to the climatic 
impacts on their own populations and territory. At the same time, some 
national governments, including ours, will need a framework for asses- 
sing worldwide consequences and policy options that are international 
in scope. 

Just as governments will assess differently the implications of 
climate change for their own countries, some perceiving gains and 
others losses, so will interests be divided within countries. Not only 
are some countries, like our own, large enough to have diverse climates 
subject to different kinds of change, but people in the same climate 
are affected differently according to how they live and earn their 
living, their age and health, what they eat, and how they take their 
recreation. Our framework has to be susceptible of disaggregation. 

The framework should be construed as moving through time. The 
changes take time; the uncertainties unfold over time; policies and 
their effects have lead times, lag times, and growth rates. Govern- 
ments and people will attach different discounts to events and 
conditions at different distances in the future. And a country that 
appears to be victim or beneficiary of a climate-change forecast for 
the next 75 years would not be helped or hurt the same amount, or 
necessarily even in the same direction, by an additional 75 years of 
the same scenario. 



1.4.3 Categories of Response 

Schelling (Table 1.11) develops a framework consisting of four cate- 
gories of response, arrayed against background climate and trends. 
Category 1 is prevention, containing options for affecting the produc- 
tion of C0 2 . Category 2 is removal: if you cannot help producing 
too much C0 2 , can you remove some? Category 3 consists of policies 
deliberately intended to modify climate and weather: if too much^CC 
is produced and not enough can be removed, so that concentration is 



58 









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Industrial emissions 
non-CO 2 greenhouse 
gases 
particulates 


Energy 
per capita demand 
fossil versus 
nonfossil 


Agriculture, forestry, 
land use, erosion 
Farming and other 
dust 
Agricultural emissions 
(N20, CH 4 ) 


Water supply, demand, 
technology, transpor 
conservation, exotic 
sources (icebergs, 
desalinization) 


^Responses may be cons 



60 

going to increase and climate is going to change in systematic fashion, 
can we do something about climate? Finally r Category 4 is adaptation, 
consisting of all the policies or actions taken in consequence of 
anticipated or experienced climate change. 

In Category 1, production of C0 2 , there are two main subdivisions: 
energy and land use. Energy breaks down into three main subdivisions: 
reduction in total energy use, reduction in the fossil fuel component, 
and switching to less carbon-intensive fossil fuels. Land use cuts 
across categories 1 and 2. There is no way to "unuse" fuel that has 
been burned, but forests can be grown or cut, and the net effect can go 
either way. Preserving a growing forest rather than cutting it can be 
thought of as producing less C02 or removing it from the atmosphere. 
The relevant land use encompasses more than forests and other living 
biomass. What happens to forests affects the release of carbon from 
the exposed soil, and so does what happens to unforested land through 
cultivation, erosion, and other disturbances or changes. 

Category 2, removal of C0 2 , shares with production the character- 
istic that it affects the global carbon inventory. Removal can be 
subdivided into processes that take C02 out of the atmosphere at 
large and those that "scrub 11 the C0 2 or otherwise remove it directly 
from the products of combustion, i.e., from stack gases and other 
exhausts. With respect to using photosynthesis and reforestation to 
reduce atmospheric C02 one conclusion is inescapable, irrespective of 
a hundred years' technological change: increasing the standing stock 
of trees can be no great part of any solution to the growing C0 2 
problem. That does not mean that a strategy for the use of lands and 
forests should ignore CO 2 , only that the role of trees, standing or 
fossilized, will be modest. "Scrubbing" from stacks and "washing" by 
the oceans offer the possibility of yielding to technological advance. 

Category 3, modification of climate and weather, can be summarized 
in four points. (1) From study of C0 2 we know that, in principle, 
modification of climate and weather is feasible; the question is what 
kinds of advances in climate and weather modification will emerge over 
the coming century? (2) Interest in C0 2 may generate or reinforce a 
lasting interest in national and international means of climate and 
weather modification; once generated, that interest may flourish 
independently of whatever is done about C0 2 . (3) Climate and 
weather modification may be more a source of international tension than 
a relief. (4) C0 2 may not dominate the subject of anthropogenic 
climate change as it does now; emission of, or reduction of emission 
of, non-C0 2 greenhouse gases may become increasingly important to 
policy on climate change. 

Category 4, adaptation, policies, or actions taken in consequence of 
anticipated or experienced climatic change, will consist of a multitude 
of largely decentralized, unconnected actions. Adaptation can be 
undertaken by units of all sizes, families, firms, ministries and 
departments, cities, states, nations, and international organizations. 
Impacts of climatic change could, of course, be numerous and diverse, 
affecting agriculture and water supply, ecosystems, and location of 
industry, for example, and adaptive response could thus take numerous 
forms, from writing assessment reports (studying the problem), to 



61 

developing markets for water and incentives for water conservation, to 
enlarging buffer stocks, to strengthening financial institutions (e.g., 
insurance), air conditioning and central heating, and educational and 
training activity. Qualitatively, the basic adaptive responses would 
seem to be learning new skills and relocation or migration 
(Meyer-Abich, 1980) . 



1.4.4 Reprise 

Overall, we find in the C0 2 issue reason for concern, but not panic. 
Although the prospect of historically unprecedented climatic changes is 
troubling, the problems that may be associated with it are of quite 
uncertain magnitude, and both climate change and increased CO 2 may 
also bring benefits. There are theory and evidence for each link in 
the chain of causal inference that we have described, but it could be 
that emissions will be low, or that concentrations will rise slowly, or 
that climatic effects will be small, or that environmental and societal 
impacts will be mild. Thus, we make some tentative suggestions about 
actual and near-term changes of policies, firmer recommendations about 
applied research and development with regard to the possibility of a 
C0 2 -induced climatic change, and strong recommendations about 
acquiring more knowledge of various aspects of the C0 2 question. In 
our judgment, the knowledge we can gain in coming years should be more 
beneficial than a lack of action will be damaging; a program of action 
without a program for learning could be costly and ineffective. In the 
words of one reviewer of the manuscript of this report, our recommenda- 
tions call for "research, monitoring, vigilance, and an open mind." 



1 . 5 RECOMMENDATIONS 

1.5.1 Can COo Be Addressed as an Isolated Issue? 

Before discussing actions and policies, we raise the question of 
whether C02 should be treated jointly with other issues or as a 
separable, isolated issue. 

If one chooses to isolate the C02 issue, it can be judged in 
basically two ways. One is the approach of welfare economics: to 
measure the potential costs of a C0 2 buildup against the potential 
costs of controlling C0 2 emissions (or other societal reponses) . For 
example, Nordhaus (1980) has proposed an optimal control strategy for 
concentrations, in which strategies are judged by the effects they 
generate on paths of consumption. "Consumption" here is interpreted in 
a broad way, including not only conventional items, such as food, 
clothing, and shelter, but also intangibles, such as enjoying the 
environment. The purpose of economic policy and C0 2 control is to 
enhance total consumption to the greatest extent. The central result 
of an analysis along these lines at present is that, given current 
knowledge, we are highly uncertain about the appropriate direction and 
stringency of CO 2 controls. The key uncertainties in the analysis 



62 

are (1) the economic and social impact of elevation of C0 2 concentra- 
tions, (2) the economic costs of controlling C0 2 emissions, and (3) 
the relevant value judgments (e.g., discount rates) that we should 
apply in weighing alternative paths. In this approach the best single 
investment strategy for coping with the CO2 issue is more research. 

A second approach that treats the CO 2 issue in isolation compares 
it with other areas in which societal investments might be made. Is 
C0 2 more or less important than nuclear war, or economic depression, 
or population growth, or drug addiction? If CO 2 comes out far down 
the agenda, the implication is that little or no investment, even of 
research, need be made in it. Meyer-Abich (1980) has argued that C0 2 
is "chalk on a white wall" or a "particular darkness in the night." He 
proposes that the world already faces such serious problems of energy, 
agriculture, water, and land use that additional problems from climatic 
change will be trivial; C0 2 will be a marginal, probably negligible, 
factor . 

If the significance of C0 2 is not isolated as an issue, the 
approach is to stress its ties to other social, economic, and environ- 
mental problems. For example, use of fossil fuels generates not only 
C0 2 but other problems for air and water quality. Deforestation 
creates not only CO 2 but problems of soil erosion. And, similarly, 
responses that might be useful with respect to C0 2 -induced climatic 
changes such as reducing water demand or increasing water supply in 
the Great Plains of the United States might be appropriate responses 
to other problems, like depletion of the Ogallala Aquifer. Attitudes 
toward C0 2 may be different if one treats it jointly with other 
issues; policies judged expensive for one issue might seem more afford- 
able as responses to a combination of issues. Advocates of action 
deriving from potential CQ 2 problems generally rely on this argument. 
A single problem, like C0 2 or acid rain or soil erosion, may not 
weigh heavily in the cost-benefit calculus, but combinations of problems 
may weigh heavily indeed. It is the essence of the political process 
to come to terms with arrays of problems. 



1.5.2 Actual and Near-Term Change of Policies 

We recommend caution in undertaking any major changes in current 
behavior and policies solely on account of CO 2 . It is probably wiser 
not to act aggressively to "solve the CO 2 problem" right now when we 
really do not know the future consequences or context of C0 2 increase. 
In trying to consider the world of 50 or 100 years from now, we cannot 
be sure that we can tell the difference between solutions and problems. 
It is instructive to look back at, say, 1905 to see if even the best 
guesses made at that time about accumulating world problems and their 
solutions were actually valid or useful as planning guides for the 
twentieth century. Life has changed since then in many unexpected 
ways; penicillin and air transport are vivid examples. 

It is not easy to anticipate now what will appear as correct 
decisions with respect to C0 2 50 or 100 years hence. For example, we 
might anticipate that climatic warming in the north would permit 



63 

seaborne commerce through the Northwest Passage. However , it is quite 
possible that transport will be so different in generations hence that 
surface ice cover in the Arctic will be largely irrelevant, and expen- 
ditures to develop Arctic transport as we can currently envision it 
would be wasteful. 

Allowing for this general caution, we nevertheless recommend that 
activities and planning that involve long time scales (i.e., on the 
order of decades or more) particularly concerning agriculture and 
water resources explicitly incorporate the assumption that the climate 
of the future is unlikely to resemble the climate of the recent past. 
In fact, because of trace gases besides C0 2 , future climate is likely 
to diverge increasingly from our recent climate whether we control CO 2 
or not. Planners of hydroelectric or irrigation systems, agricultural 
infrastructure, forestry, hazardous-waste disposal, nature conservation, 
and other areas might ask whether current policies are vulnerable to 
climatic change and whether alternative policies can be designed that 
will both serve our needs and be robust in the face of climatic change. 

The need to design and manage on the basis of likely future climates 
as well as past climates is probably greatest in the area of water 
resources. Experience has shown that the planning and construction of 
water-resource developments in major river basins can take several 
decades. It is not too soon to begin to think of ways in which the 
planned use of water could alleviate potential effects of climatic 
changes or even take advantage of them. Several possible measures come 
to mind: changes in legislation that would allow water to be trans- 
ferred from one river basin to another; improved efficiency in the use 
of water for irrigation; conservation of waste water and of municipal 
water supplies; limitations on the size of irrigated areas; increases 
in crop yields per unit volume of applied water; and enhancement of the 
recharge of aquifers (Revelle, 1982). 

In fact, greater efficiency in water use is a prudent goal to pursue 
even if we neglect C02 concerns. Opportunities for conservation of 
agricultural water through improved conveyance and farm distribution 
systems, application methods, scheduling, crop selection, and cropping 
practices are great. So, too, are the opportunities to halt and reverse 
the degradation of irrigated lands beset by waterlogging and saliniza- 
tion (White, 1983) . 

In fact, as Waggoner (this volume, Chapter 6) shows, we already know 
how to reduce greatly the harm of drought by storing more precipitation 
and getting more yield from the stored water. Fewer weeds and tillages 
decrease the loss of soil moisture, while more stubble captures more 
precipitation. Other means of increasing storage include barriers that 
catch snow, leveling and terracing to decrease runoff, and harvesting 
water from nearby acreage. Matching irrigation to need can decrease 
pumping; but if most of the former excess was returned to the ground- 
water supply, the saving of water will not be great. Getting more 
yield per acre increases the yield of marketable product per unit of 
water consumed. Changing to shorter growing-season crops can also 
increase water-use efficiency. 

Concern about climatic changes may foster readjustments in agricul- 
ture and water management that will be beneficial in any event. For 



64 

example, some of the same measures that would help us to prepare for a 
permanently drier average climate such as encouragement of water con- 
servation or provision of additional carry-over water storagewould 
make current agriculture and water use more resistant to passing 
droughts . 



1*5.3 Energy Research and Policy 

The major impetus to this report was concern about the projected impact 
on atmospheric C02 of fossil fuel combustion, coal conversion, and 
related synthetic fuels activities authorized in the Energy Security 
Act of 1980 (PL-96-294, Title VII, subtitle B, appended) . During 1979 
many scientists expressed concern that U.S. energy policy and its 
changing emphasis would exacerbate the C02 problem. They recommended, 
in part, conservation of fossil fuels and choice among fossil fuels and 
conversion processes based on C0 2 emissions, implying a bias against 
coal and carbon-based synthetic fuels. 

Our study does not support the extent of concern expressed about the 
importance of decisions planned at that time about synthetic fuels in 
relation to the C0 2 issue (see the following section for more detail) . 
Analysis of the contribution of various factors to uncertainty about 
future emissions of C0 2 shows that other factors besides the choice 
among fossil fuels are much more important in determining future emis- 
sions. If one wishes to reduce emissions through decisions relating to 
energy policy, placing a moratorium or limitation on development of 
synthetic fuels, especially in one country, even a large one like the 
United States, is likely to be a poor choice. By itself, shifting the 
fuel mix within fossil fuels is highly unlikely to achieve a reduction 
of C02 concentrations by more than a few parts per million by the 
year 2100, even if accomplished globally. A global policy banning 
carbon-based synfuels might lead to a reduction in C0 2 concentration 

suDol?^ i PPm ^ ? e ye " 21 ' because of the lar * e alternative 
n" P Y K T a Y ailable ' Suggestions that a near-term shift to 
carbon-based synfuels could advance the time of C0 2 doubling by 

S^rSL^f'T 9 " 11 ^; Ur StUdY d eS ' h Wever ' W-t that 



Thus, we conclude that 



rapidly, that the fraction remaining airborn! ^ nS are rising 
very sensitive to CO, increase or LS JS * 1S 19h ' that climate is 
are costly and divisive In such a caL ***** Cliniate change 
ability to maRe a transition to nonfos^ f^?* * ^ a 



65 

2. The potential disruptions associated with CO 2 - induced climatic 
change are sufficiently serious to make us lean away from fossil fuel 
energy options, if other things are equal. However, our current assess- 
ment of the probability of an alarming scenario justifies primarily 
increased monitoring and vigilance and not immediate action to curtail 
fossil fuel use. 

3. Analysis of prospective 002 emissions does not offer a strong 
argument for making choices among particular patterns of fossil fuel 
use at this time. 

It should be kept in mind that important uncertainties about future 
emissions stem from variables such as rates of productivity and 
population growth, as well as from decisions centered in the energy 
sector. Several of these variables that affect future emissions, like 
productivity growth, population growth, or technological change, 
strongly resist adjustment by policymakers. 



1-5.4 Synfuels Policy and C02 

By synfuels policy we refer to the development and use of new carbon- 
based fuels derived from fossil sources to replace a portion of the 
existing fuel mix. Synfuels would include oil and gas from coals and 
shales. At present, synfuels are a negligible part of world energy 
supply. 

Overall, some skepticism is in order about the relation between the 
CO 2 issue and encouragement of synfuels. The causal links are 
complex and numerous; the direction of the effect is ambiguous; the 
importance of the effect has not been convincingly demonstrated. As 
suggested above, it would probably be more effective to address the 
issue of C02 emissions through policies other than the highly 
uncertain mechanism of slowing or speeding synfuels development. 

Synfuels policy might be separated into three components: 

1. Short-run emergency preparedness (for example, stockpiles or 
temporary surge capacity) . 

2. Tax or subsidy arrangements for research and development (R&D) 
on synfuels or for the use of synfuels. 

3. Less specifically focused policies that might have an effect on 
synfuels and 002 emissions (mass transit, hydrogen R&D, nuclear- or 
solar-power policy) . 

A range of interactions can be envisaged between the different kinds 
of policies and CO2. From short-run policies in one or a few nations, 
such as strategic stockpiles, there is unlikely to be significant impact 
on 002* More widespread, enduring tax and subsidy arrangements are 
likely to have more important effects on CO 2 , but the impacts are not 
obvious. Of course, taxes on carbon-based fuels are the most predict- 
able in their emission-reducing impact. 

The most likely kind of synfuels policy is to encourage R&D and com- 
mercialization of synfuels; this was the major goal of the 1980 Energy 



66 

Security Act. An example of such a policy is an interest guarantee f 
which acts as an implicit subsidy to production and as a subsidy to 
learning about the technology. The effect of such a subsidy is subtle. 
If in fact the technology is viable , the subsidy will speed up its 
introduction, but it is unlikely to have a major impact on C0 2 
emissions in the long run. If the technology is not viable/ it will/ 
like commercial supersonic transport, tend to disappear even with 
subsidies. 

A second subtlety arises because the impact of early introduction of 
synfuels on CO 2 emissions is ambiguous and could either speed up or 
slow down 002 emissions. For example, if the price of the synfuel 
turns out to be high and the availability of other fuels is restricted, 
demand for energy may fall, and CO2 emissions may decline. If, on 
the other hand, the new synfuel is attractive economically, it may lead 
to greater CC>2 emissions. Another ambiguity arises because the 
synfuel may replace either a carbon-based or noncarbon-based fuel. In 
the former case, it might have a small impact on C0 2 emissions; in 
the latter, a larger one. It is impossible to determine a priori the 
net effect of such influences, but using energy models might provide a 
range of answers. 

Probably the most important determinant of the role of synfuels will 
be their interaction with other policies. Thus, a policy that encour- 
ages mass transit or solar energy could discourage carbon-based syn- 
fuels. A nuclear moratorium or heavy regulation of nuclear power could 
encourage synfuels. A stringent environmental policy, banning surface 
mining or growth in emissions of air pollutants, could discourage 
synfuels. These more subtle influences are probably important in 
determining CC>2 emissions from synfuels, more important than synfuels 
policy itself. 



1.5.5 Applied Research and Development 

The prospect of climate change clearly lends urgency to applied research 
and development in two areas besides energy: agriculture and water 
resources. Although no detailed timetable of climatic change is yet 
available, we have some notion of the general character of the climatic 
challenges that may be ahead of us. America is fortunate in possessing 
widespread and effective research networks in both agriculture and 
water resources. The tasks are to build and to maintain a strong and 
flexible national capability to adapt to changing climate and indeed to 
exploit new opportunities that changed climates may offer. 

Modern agriculture possesses great flexibility and adaptability to 
change. For example, changing crops can be swift. And changing the 
variety of a crop by planting a different strain can be even swifter, 
because little in the process needs to be altered, from the dealer who 
supplies chemicals, to the farmer who must finance equipment, to the 
consumer who may be scarcely aware of the change. The question is 
whether breeders can develop new varieties to adapt to climate as fast 
as it changes. A complete breeding cycle is approximately a decade 
from the beginning of inbreeding to the marketing of a product; the 



67 

objectives can be shifted during the first 5 years, and new hybrids 
emerging are those adapted to the final 3 years. Ideally, one would 
identify the critical environmental changes for a given crop and 
engineer appropriate genetic changes to cope with the new conditions. 
This process is as yet beyond our capabilities, but research may bring 
it within our grasp. In any event, there is ample reason to believe 
that steady work by plant breeders will continue to produce varieties 
of the life-sustaining major crops which are adapted to a changing 
environment. 

There are also numerous avenues to pursue if there is less rainfall 
and runoff. White (1983) emphasized two sets of advanced techniques 
with potentially great impact and broad application: those relating to 
groundwater exploration and extraction and those relating to re-use of 
water. Improved seismic and geological surveys, well drilling, and 
pumping methods are opening up a huge volume of water previously 
ignored or inaccessible. Where this water exists in aquifers that are 
easily rechargable, it represents a potentially permanent addition to 
water supplies. White pointed out that the techniques have been of 
major importance in developing countries that can use them to gain 
access to previously untapped supplies without building elaborate 
storage and conveyance works. As a result of advances in treatment 
methods and in system planning, the re-use of water is also beginning 
to be viewed as a practical measure in both urban and agricultural 
settings. Other more tested alternatives deserve more widespread 
appraisal as well; these include water-pricing policies, leak detection, 
water-conserving devices, canal lining when seepage from the canal 
enters a saline-water aquifer, water application scheduling, drip 
irrigation, and choice of water-efficient or salt-tolerant crops. 

Water technology needs to be developed complementary to changes in 
social and economic institutions. Research and activities should not 
focus exclusively on water control but on the combination of water 
development, land-use management, and economic and social adjustment. 



1.5.6 Basic Research and Monitoring 

The C0 2 issue involves virtually every branch of science and impinges 
on virtually every area of human activity. Whether motivated by 
concern for C0 2 or some other problem, or simply by scientific 
interest, research is in progress on nearly every identifiable topic 
that could contribute to our understanding. Research is funded by 
several federal agencies and coordinated through the National Climate 
Program and the lead agency for CO 2 -related research, the Department 
of Energy (DOE) . The DOE Carbon Dioxide Research and Assessment 
Program itself supports a broad program of investigation specifically 
aimed at the periodic development of detailed and comprehensive tech- 
nical assessments drawing on the work of a large community. Indeed, in 
developing some portions of this report, we have worked closely with 
the DOE program and drawn on its participants, research results, and 
periodic coordination and review activities. 



68 

As we have emphasized throughout this report, the uncertainties 
regarding the C0 2 issue are numerous. Some areas of uncertainty are 
unlikely to diminish rapidly. The issue, and research directed at its 
illumination, will be with us for a long time. While we have tried to 
carry out some exemplary analyses in this report, we can propose no 
fresh and efficient route to the knowledge we would like to have. 
Rather than stress the need to attack any particular link in the C0 2 
argument, we stress the need for balanced attention to the major com- 
ponents: emissions, concentrations, climate change, and environmental 
and social impacts and responses. A plethora of research recommenda- 
tions exists, and several areas, for example, the carbon cycle and 
climatic effects, are being pursued with considerable vigor. Other 
areas may require more concentrated effort. For example, detection of 
the effects of increasing C0 2 may require better focus and increased 
interaction among climatologists, statisticians, investigators from 
several other disciplines, and those responsible for design and opera- 
tion of monitoring programs. We offer several general comments about 
research with regard to C0 2 assessment before addressing specific 
subject areas. 

1.5*6.1 General Research Comments 

1. A broad, healthy program of basic research in the physical, 
biological, and social sciences is an indispensable foundation for our 
efforts to understand the C0 2 issue, which offers vivid evidence of 
the indivisibility of basic and applied research. The kinds of knowl- 
edge that we would like to have for analyzing the C0 2 issue 
knowledge, for example, about the role of clouds and the ocean in the 
climate system or about the behavior of ice sheets are unlikely to be 
produced on a procurement schedule defined by contracting agencies. 
While rates of learning appear to be rapid in most of the areas of 
concern for the C0 2 issue, fundamental, difficult questions are 
involved, and in some areas we simply do not know whether or when 
insights will be forthcoming. 

2. The importance of geophysical and biospheric monitoring must be 
stressed. Over the past decade or so we have seen a series of climate- 
related issues become prominent and subjected to costly analysis. These 
include, for example, the Sahelian drought, the impacts of stratospheric 
flight on the ozone layer, and now C0 2 . In each case, data bases are 
deficient, whether about the variability of climate in the Sahel, or on 
concentrations of gases in the stratosphere, or of carbon inventories, 
global temperatures, and C0 2 concentrations. With sound, stably 
supported, long-term geophysical and biospheric monitoring programs we 
can have the capability for dealing with such issues and with new ones 
that will undoubtedly arise in the future. 

Certain kinds of routine data collection are both expensive and 
uninteresting and may not appeal to the most gifted scientists, yet 
their cumulative significance over time can be important. There is a 
problem of how to ensure that an adequate investment is made in this 
kind of data collection and that it is done well with methods that are 



69 

validated by the most knowledgeable people. Historically, this kind of 
deficiency has been serious with respect to research programs in many 
environmental areas (Brooks, 1982). 

3. More systematic setting of research priorities based on the 
relative contributions of various problems to the overall uncertainty 
of the C0 2 issue should be considered. For example, the sensitivity 
studies of Nordhaus and Yohe (this volume, Chapter 2, Section 2.1) and 
Machta (this volume, Chapter 3, Section 3.6) are suggestive about 
specific research priorities in certain areas. We cannot, it should be 
emphasized, move directly from estimates of sources of uncertainties to 
a budget allocation for research funds on C0 2 . It may be easier, for 
example, to reduce uncertainties about the "depletion factor" for carbon 
fuels than about future "productivity growth," even though the latter 
appears to be a greater source of uncertainty. The problem is to pro- 
vide incentives that will induce the most gifted researchers to shift 

to the most significant problems, i.e., to those areas contributing the 
most to uncertainties regarding future impact of C0 2 . But these 
researchers must be convinced not only by the social importance of the 
problem but also by the perception of genuine scientific opportunities 
to contribute to its resolution. 

4. The question of overall program balance in the C0 2 area also 
needs to be considered. We argued above that the way an issue is 
defined is fundamental to what research and policies are considered and 
that the main perspective here should probably be climate change, not 
simply C0 2 . A C0 2 program geared heavily toward estimation of C0 2 
concentrations and associated warming will probably be less useful than 
one with strong interests in, for example, other greenhouse gases, water 
resources, and sea-level rise, as well. Indeed, it may be wise for the 
government to anticipate the evolution of concern in this area into a 
broader program on human activities-greenhouse gases-climate change- 
adaptation. 

5. Human-induced climatic change and the CO 2 issue more generally 
are not problems for which traditional paradigms of policy analysis and 
methods of assessment, like those taken from economics, engineering, 
and decision analysis, have had much success (Glantz et al., 1982). It 
is naive, indeed mistaken and misleading, to expect a "definitive" 
assessment of the C0 2 issue here, or from other groups or individuals 
in the foreseeable future. What is required is a sustained approach, 
probably no larger than the sum of efforts currently under way, which 
emphasizes carry over of learning from one effort to the next. 

A healthy reaction to C0 2 would be one in which there is a steady 
production of knowledge, and in which every few years, either in the 
United States or in another country or in a combination of countries, 
developed or developing, east or west, some group undertakes a thought- 
ful synthesis. As we become generally better at global geophysical and 
biospheric modeling and at thinking one or two or more generations into 
the future, assessment of the C0 2 issue will deepen and strengthen. 
Each assessment must be regarded as part of an iterative process, con- 
sisting of efforts to assemble relevant knowledge, to assess the 
problem based on the assembled knowledge, to estimate the marginal 
value of additional knowledge to reduce uncertainties in the 



70 

assessment, and to diffuse results to individuals and groups with power 
to act (Mar, 1982). 

6. While the C0 2 issue remains appropriately so, in our view 
largely in the research community, it is important to consider the 
possibility that the issue may become prominent in political arenas. 
It is quite possible that, as a result of weather conditions or bad 
harvests in one part or another of the world, C0 2 will abruptly rise 
nearer to the top of many national and international agendas, regardless 
of the scientific basis for concern. How long it might hold such a 
position one can only speculate; most issues star for only a short 
while. And it is unlikely that prominence for one session of the 
United Nations or the United States Congress would bring about policy 
changes that would be of lasting importance with respect to C0 2 . It 
is important that we fashion perspectives and programs that can be 
sustained through periods of excessive attention or inattention to the 
issue. 



1.5.6.2 The International Aspect 

Should it be desirable to control C0 2 emissions, it would be natural 
to suppose that they could be controlled in a way similar to conven- 
tional pollutants, but this supposition would be optimistic. Most 
externalities, or side effects of economic activity, are at least 
internal to nations; thus a government can weigh the costs and benefits 
of a control program and decide that, on balance, it is in the inter- 
ests of its citizens. The C0 2 problem is different from conventional 
pollutants because it is an externality across so much space and time. 
Thus, just as we as individuals have little reason to curtail our 
emissions, we as a nation have little incentive to curb CO 2 emissions. 
By curbing our C0 2 output, we make little contribution to the solution 
and do not know whether we will receive any benefits. With respect to 
a C0 2 -induced climate change, there is little incentive to act alone. 
The problem is exacerbated by the long time period over which CO 2 can 
affect our society and the environment. Although politicians may have 
one eye on posterity, political systems tend to be myopic and to 
emphasize short-term rewards. 

Given the need for widespread, long-term commitment, a C0 2 control 
strategy could only work if major nations successfully negotiated a 
global policy. While such an outcome is possible, there are few 
examples where a multinational environmental pact has succeeded, the 
nuclear test ban treaty being the most prominent. Other clearly recog- 
nized problems whale fisheries, acid rain, undersea mining, the ozone 
layer emphasize how time on the order of decades is required to 
achieve even modest progress on international management strategies. 

With regard to C0 2 increase, the multilateral bargaining is 
severely complicated by the likelihood that some major countries will 
probably benefit, at least from a moderate rise. For example, it is 
sometimes conjectured that the Soviet Union and Canada would benefit 
from a warmer climate. Given that these two countries (and the former's 
allies) burn 25% of world coal and hold a larger share of carbon 



71 

resources in the ground/ it is hard to see how a C0 2 control strategy 
can succeed without them* Given the unlikelihood that the United 
States or other western nations will compensate the Soviet Union for 
participating, it is hard to see why the Soviet Union would participate* 
If a major nation or group of nations does not participate, it is 
difficult to envisage others, particularly developing countries, making 
a major sacrifice. Thus, differences in the experience and expectations 
of nations pose major obstacles to any international agreement for 
control of C02- 

While we may not be optimistic that agreement could ever be reached 
among nations about a control strategy for C(>2 (should such a control 
strategy be the desirable response in the first place) , it is essential 
that research and dialogue about the CO 2 issue be carried on inter- 
nationally. Study of world energy supply and demand inherently neces- 
sitates data from many countries, and a variety of national and 
international views of the energy situation is likely to enrich our 
analysis substantially. Study of the carbon cycle is also inherently 
global, and significant capability in this area, moreover, resides 
outside the United States. The atmosphere and oceans are similarly 
global, and in many countries there are strong research communities 
investigating them. Finally, monitoring of the climate, the bio- 
sphere, the oceans requires international participation. Exchange 
between nations and international collaboration should therefore be 
pursued extensively with respect to study of the CC>2 issue. 

In addition, no matter how outstanding the analysis coming out of a 
particular country might be, other countries will always view individual 
national studies with suspicion. With a potentially divisive issue 
like C0 2 , it is critical that assessments be undertaken independently 
by several nations, as well as by relatively neutral international 
groups. We commend the cooperation that has been initiated among the 
International Council of Scientific Unions, the World Meteorological 
Organization, and the United Nations Environment Program to prepare a 
careful assessment of the C02 issue during the next few years. The 
United States should contribute energetically to this effort from both 
governmental and nongovernmental communities concerned with studying 
and responding to the prospect of a CC>2- induced climate change. 

It is worth noting that on particular issues, like detection of a 
CO2~induced climatic change and evaluation of climate model results, 
there may be considerable efficiency in having an international focal 
point. Qualified individuals and groups from the United States should 
participate in support of such centers and assist where possible and 
appropriate in sharing, comparing, and analyzing findings on important 
questions. We note that in several areas for example, biogeochemical 
cycles, atmospheric monitoring, and climate research international 
programs and organizations have been functioning well. 

In general, diffusion of sound information and calm, thoughtful 
anticipation of the future may be the best international insurance 
against poor decisions about possible control of C0 2 emissions, 
against possible adverse consequences of climatic change, and against 
the issue's becoming a source of major conflict. Indeed, if approached 
in a constructive manner, the 002 issue offers an opportunity to 



72 

strengthen international cooperation and capabilities in many highly 
desirable respects. Vigorous efforts should be made to prevent 002 
from becoming politically divisive; instead the nations of the world 
should seek to benefit from it as a catalyst for learning how to treat 
common problems effectively* 



1.5.6.3 Projecting C0 2 Emissions 

The current modeling and knowledge of future C0 2 emissions appears 
marginally adequate today; we have a general idea of likely future 
trends and the range of uncertainty. It may be that further effort 
could increase the accuracy of our forecasts substantially. Given the 
large uncertainty that future energy growth and energy projections are 
contributing to the CO2 issue, this area may well merit more research 
attention and support than it has received in the past. Future research 
efforts might be designed with four points in mind. 

1. In general/ the most detailed and theoretically based projections 
of CO 2 emissions have been a spillover from work in other areas, 
particularly energy studies. This fact suggests that continued support 
of energy modeling efforts will be of importance in pushing out the 
frontier of knowledge about future CO 2 emissions, as well as the 
interaction between possible C0 2 controls and the economy. 

2. We have identified a serious deficiency in the support of 
long-run economic and energy models in the United States. There is not 
one U.S. long-range global energy or economic model that is being 
developed and constantly maintained, updated with documentation, and 
made usable to a wide variety of groups. This shortcoming is in con- 
trast to climate or carbon-cycle models, where several models receive 
long-term support, are periodically updated, and can be used by outside 
groups. Another contrast is with short-run economic models, which are 
too plentiful to enumerate. 

3. Most CO 2 projections have been primitive from a methodological 
point of view. Work on projecting C0 2 emissions has not drawn suf- 
ficiently on existing work in statistics, econometrics, or decision 
theory. There has been little attention to uncertainties and proba- 
bilities. Also, considerable confusion of normative and positive 
approaches exists in modeling of C0 2 emissions. 

4. Application of models for analysis of policies, where there are, 
for example, feedbacks to the economy from climatic change or CO 2 
control strategies, is just beginning. Efforts to evaluate the 
effectiveness for C0 2 control of energy policies of particular 
nations or groups of nations in a globally consistent framework have 
been lacking. 



1.5.6.4 Projecting C0 2 Concentrations 

Projecting C0 2 concentrations consists essentially of the application 
of our knowledge of the carbon cycle to projections of C0 2 emissions. 



73 

After more than a decade of intense research, findings on the roles of 
the different reservoirs of carbon f particularly the biosphere and the 
oceans, remain in obvious, unresolved conflict. Efforts to improve our 
understanding of the carbon cycle are desirable for many reasons, and 
monitoring of carbon in its various forms must be maintained and in some 
cases expanded. However, examination of uncertainties in the CO 2 
issue introduced by different factors suggests that uncertainty about 
the airborne fraction is of less significance for the overall C(>2 
issue than the extensive discussion in the C0 2 literature would 
suggest. Questions that may merit more attention in the carbon cycle 
area include the history of C02 concentrations and the future behavior 
of the oceans and biosphere beneath a high-C0 2 atmosphere. 

With respect to the biosphere, surveys, field experiments, and 
dynamic models will be the means to achieve insights into the responses 
of the biota to increasing CC>2 concentrations and changing climate. 
To provide a better basis for this work, inventories of carbon in the 
biosphere should be improved; satellite surveillance may be particu- 
larly helpful in strengthening the empirical basis of biospheric 
research and in assessing biospheric changes on a regular basis. 
Clarifying the history of the size of the biotic pool over the past 
century is also useful; it may help corroborate data on the preindus- 
trial concentration, reduce uncertainty about the fraction of emissions 
remaining airborne, and provide a check on the quality of carbon-cycle 
models . 

Historical and geological data on C0 2 from records of the past 
such as ice cores have proven to be valuable, and an expanded effort to 
confirm and refine previous findings should be undertaken. Recent 
ice-core data have placed new constraints on what the atmospheric C0 2 
levels could have been in past centuries. Isotopic studies of tree 
rings, lake sediments, and current air samples offer the potential to 
elucidate further the history and fate of atmospheric C02. 

With respect to the atmosphere, the need to continue high-precision 
observations cannot be overemphasized. The atmospheric CC>2 data 
provide information beyond simple evidence that atmospheric CC>2 is 
increasing. Through careful measurements, one is able to derive 
valuable information from the temporal and spatial variability of 
CO2- For example, the pattern of results so far is suggestive of a 
minimal contribution from sources of C0 2 other than fossil fuel over 
the past couple of decades or even that the biota are a net sink for 
CC>2, although the limited quantity of the data (and the possibility 
of alternative explanations) prevent any definitive statement today 
that excludes nonfossil fuel sources. 

With respect to the oceans, it now appears to be quite possible to 
measure the changing C02 properties of the ocean over time by using 
modern techniques, though no ongoing program yet exists to do so. 
Previous programs have provided regional coverage in different years 
and seasons. We recommend initiation of a program with more con- 
sistency in space and time. Until quite recently, oceanic C0 2 system 
measurements contained substantial inaccuracies. 

Although many oceanographers believe that current methods appear 
satisfactory to answer approximately the question of how much C02 the 



74 

sea takes up, there is room for improvement. Models of ocean CO 2 
uptake have depended greatly on tracer data, particularly natural and 
bomb-produced ^*C, tritium, and, more recently, halogenated hydro- 
carbons. Typically, these models represent some features of ocean 
chemistry quite well, even though they represent vertical transport by 
a simple diffusion coefficient. The models treat only the CO 2 
perturbation and do not yet adequately mimic the natural and complex 
C0 2 -oxygen nutrient biogeochemical cycles within the ocean. As our 
ocean data base grows, the current generation of one-dimensional models 
will become increasingly inadequate, and incorporation of the CO 2 and 
tracer data into new models will be required. Warming accompanying 
atmospheric C0 2 rise will also affect the ocean. Storage of heat in 
the upper layers will postpone, but not prevent, climate change. Models 
of this heat storage generally treat it as passive uptake, not affecting 
water mass formation and vertical circulation. If changes occur in 
ocean transport and dynamics, they may affect ocean C0 2 uptake in 
significant ways. For example, changes in formation of oceanic bottom 
waters in high latitudes may affect the rate of transfer of dissolved 
CO 2 to the deep ocean. Progress in incorporating such processes and 
features into more advanced ocean models is anticipated and should be 
encouraged. Such models will require complex global measurements from 
ocean research ships, ocean-scanning satellites, and other sources. 

With respect to the methane hydrate clathrates, we recommend further 
consideration of the probable effects of a rise in ocean-bottom 
temperatures on the stability of the clathrates. We also recommend a 
sediment sampling program on continental slopes to determine the depth, 
thickness, and distribution of methane hydrate clathrates, especially 
where oceanfloor temperatures and depths are such that methane release 
is possible from ocean warming during the next century. 

Finally, more attention should be given to interactions among the 
biogeochemical cycles of carbon, sulfur, nitrogen, and phosphorus (Bolin 
et al., 1983). These interactions may offer another example of an area 
where there are important, as yet unforeseen, feedbacks. To illustrate, 
lower atmospheric C0 2 concentrations 10,000 to 20,000 years ago may 
have resulted from changes in oceanic biologic production, perhaps 
related to larger quantities of ocean nitrogen and phosphate. 

1.5.6.5 Climate 

The width of the range of projections associated with a given C0 2 
increase and the desire for more detailed information on prospective 
climate change warrant continuing support for climate research. 
Special attention should be given to the role of the oceans and clouds, 
model comparison and validation, extremes, and non-C0 2 greenhouse 
gases. 

The heat capacity of the upper ocean is potentially great enough to 
delay by decades the response of climate to increasing atmospheric 
C0 2 , as modeled without it; and the lagging ocean thermal response 
may cause important regional differences in climatic response to 
increasing C0 2 . The role of the ocean in time-dependent climatic 



75 

response deserves special attention in future modeling studies , stres- 
sing the regional nature of oceanic thermal inertia and atmospheric 
energy-transfer mechanisms. Progress in understanding the ocean's role 
must be based on a broad program of research and ocean monitoring. 
Particular attention should be paid to improving estimates of mixing 
time scales in the main thermocline. 

Cloud amounts, heights, optical properties, and structure may be 
influenced by CC^-induced climatic changes. In view of the uncertain- 
ties in our knowledge of cloud parameters and the crudeness of cloud 
prediction schemes in existing climate models, it is premature to draw 
conclusions regarding the influence of clouds on climate sensitivity to 
increased CO 2, particularly on a regional basis. Empirical approaches, 
including satellite-observed radiation budget data, are an important 
means of studying the cloudiness-radiation problem, and they should be 
pursued. 

Simplified models permit economically feasible analyses over a wide 
range of conditions. Although they can provide only limited informa- 
tion on local or regional effects, simplified models are valuable for 
focusing and interpreting studies performed with more complete and 
realistic models. 

In addition to improvement and validation of models, we recommend 
more research into the question of statistical properties of a warmer, 
CO 2 -enriched atmosphere. Particular attention should be given to 
possible changes in the character and frequency of extreme conditions 
and to severe storms. Variance studies with general circulation models 
(GCMs) are one potentially useful approach. Insights into the question 
of extremes may also be obtained from research in other fields, such as 
hydrology. 

With respect to non-C02 greenhouse gases, improved, sustained 
monitoring is called for. Available instrumentation and methods are 
probably sufficient for high-quality data collection. 

Equally important is the goal of obtaining an understanding of the 
mechanisms by which the gases increase. For the chlorofluorocarbons 
and several other potentially significant trace gases, only industrial 
production statistics are needed to establish annual emissions. For 
CH^ and ^0, biological sources probably dominate. Microbial 
processes in soils, water bodies, and living organisms produce and 
release these gases and a number of others. (Release of CH4 and 
higher hydrocarbons from clathrates may become significant in the 
future.) Key sites for emissions and processes have been identified, 
but a variety of field and laboratory measurements requires implementa- 
tion. Similarly, there is need for more elucidation of the mechanisms 
that control levels of ozone in the troposphere, where increases of 
ozone can have a warming effect. Interactions among the gases and with 
changing climate must be considered. 

There is also need to make careful new projections of future emis- 
sions of non-C(>2 greenhouse gases. Projections of future emissions 
of these non-CC^ gases are generally at a more primitive stage than 
are C0 2 projections. Projections have typically been derived from 
simple assumptions of linear increase or exponential growth based on a 
short segment of recent years. The times in the future vary to which 



76 

the relevant studies of agricultural production/ smelting/ industrial 
use of chemicals/ and other activities extend/ and the assumptions 
employed vary as well. It would be desirable to have studies of the 
greenhouse effect that use assumptions consistently in generating both 
C02 emissions and emissions of other infrared-absorbing trace gases. 
In human activities like those relating to emissions of the chloro- 
fluorocarbons where rapid technological change is occurring and where 
less inertia is imposed by a large and expensive capital stock than in 
the energy system/ projections for several decades are especially 
hazardous. 

The spectroscopic parameters of several of these gases are not well 
known/ and even the band strengths of some have not been measured. The 
spectral transmittance and total band absorptions also need to be more 
closely determined. These improvements will help in developing more 
accurate radiative-transfer models and in answering questions about 
band overlap between constituents and with water vapor. Such 
information is needed for defining more accurate parameters in climate 
models . 



1.5.6.6 Detection and Monitoring 

In view of the importance of verifying the theoretical results about 
climatic effects of C02/ a careful/ well-designed program of 
monitoring and analysis must proceed. The information obtained will 
help us not only in detecting C02 - induced changes as early as 
possible but in improving/ validating, and calibrating the climate 
models employed for prediction of future changes. 

If/ as expected/ the C0 2 signal gradually increases in the future/ 
then the likelihood of perceiving it with an appropriate degree of 
statistical significance will increase. Given the inertia created by 
the ocean thermal capacity and the level of natural fluctuations/ we 
expect that achieving statistical confirmation of the (^-induced 
contribution to global temperature changes so as to narrow substan- 
tially the range of acceptable model estimates may require an extended 
period. Improvements in climatic monitoring and modeling and in our 
historic data bases for changes in C0 2 / solar radiance/ atmospheric 
turbidity/ and other factors may/ however, make it possible to account 
for climatic effects with less uncertainty and thus to detect a C0 2 
signal at an earlier time and with greater confidence. 

A complicating factor of increasing importance will be the role of 
rising concentrations of greenhouse gases other than C0 2 . While the 
role of these gases in altering climate may have been negligible up to 
the present/ their significance is likely to grow. It will be dif- 
ficult to distinguish between the climatic effects of C02 and those 
of other radiatively active trace gases. Their expected relative 
contributions to climatic change will have to be inferred from model 
calculations and precise monitoring of radiation fluxes. 

A monitoring strategy should focus on parameters expected to respond 
strongly to changes in CC>2 (and other greenhouse gases) and on other 
factors that may influence climate. Candidate parameters may be 



77 



identified, their variability estimated, and their evolution through 
time predicted by means of climate model simulations. Through analysis 
of past data, continued monitoring, and a combination of careful 
statistical analysis and physical reasoning, the effects of CO 2 may 
eventually be discerned. 

Monitoring parameters should include not only data on the CO2 
forcing and the expected climate system responses but also data on 
other external factors that may influence climate and obscure C0 2 
influences. Climate modeling and monitoring studies already accom- 
plished provide considerable background for the selection of these 
parameters. Since fairly distinct climate changes are expected to 
become evident only over one or more decades, monitoring for both early 
detection and more rapid model improvement should be carried out for an 
extended period. Parameters may be selected for early emphasis on the 
basis of the following criteria: 

1. Sensitivity. How do the effects exerted on climate by the 
variables or the changes experienced by the variables on decadal time 
scales compare with those associated with corresponding changes in 

CO 2 ? 

2. Response characteristics. Are changes likely to be rapid enough 
to be detectable in a few decades? 

3. Signal-to-noise ratio. Are the relevant changes sufficiently 
greater than the statistical variability to be measured accurately? 

4. Past data base. Are data on the past behavior of the variable 
adequate for determining its natural variability? 

5. Spatial coverage and resolution of required measurements. 

6. Required frequency of measurements. 

7. Feasibility of technical systems. Can we make the required 
measurements? 

Initial application of these criteria leads to this list of 
recommended variables for monitoring: 



Monitoring 
Causal Factors by 
Measuring Changes 



in 



CC>2 concentrations 
Volcanic aerosols 
Solar radiance 
"Greenhouse" gases 

other than CO 2 
Stratospheric and 

tropospheric ozone 



Monitoring 
Climatic Effects by 
Measuring Changes in 

Troposphere/surface 

temperatures ( including 

sea temperatures) 
Stratospheric temperatures 
Radiation fluxes at the top 

of the atmosphere 
Precipitable water content 

(and clouds) 

Snow and sea- ice covers 
Polar ice-sheet mass balance 
Sea level 



78 

In the above list, evaluated more thoroughly in Chapter 5, emphasis 
has been given to parameters that may contribute, either directly or 
through model improvements, to detection of C(>2 effects at the 
earliest possible time. Over the long run, it is important to build up 
a relatively complete data base of possible causes and effects of 
climate change and characteristics of climate variability, not simply 
for detection but to assist in research on and calibration of models of 
the climate system. Once we become convinced that climate changes are 
indeed under way, we will seek to predict their future evolution with 
increasing urgency and with increasing emphasis on parameters of 
societal importance (e.g., sea level and rainfall). We should thus 
anticipate that a detection program will gradually evolve into a more 
comprehensive geophysical monitoring and prediction program. It should 
be emphasized that the strategy proposed here is a simple tentative 
step in what must be an iterative process of measurement and study. 

Collection of the desired observations will require a healthy global 
observing system, of which satellites will be a major component. 
Satellites can provide or contribute to long-term global measurements 
of radiative fluxes, planetary albedo, snow/ice extent, ocean and 
atmospheric temperatures, atmospheric water content, polar ice-sheet 
volume, sea level, aerosols, ozone, and trace atmospheric components; a 
well-designed and stable program of space-based environmental obser- 
vation is essential if we are to monitor the state of our climate. 
Requirements and technical systems for monitoring high-priority C0 2 
variables are summarized in Table 5.1. 

We will also have to continue to improve climate models to reduce 
the uncertainties in predictions of climate effects and to validate the 
models against observations, although we believe that current climate 
models are sufficiently sound and detailed to enable us to identify a 
set of variables that could form the basis for an initial monitoring 
strategy. Statistical techniques for assessing the significance of 
observed changes may have to be improved so as to deal with the charac- 
teristics of the monitored variables. In the end, confidence that we 
have detected the effect of C0 2 will have to rest on a combination of 
both statistical testing and physical reasoning. 



1.5.6.7 Impacts 

1.5.6.7.1 Sea Level and Antarctica 

With respect to sea-level rise, we recommend research to confirm the 
estimate of a possible 70 -cm rise over the next century (Revelle, this 
volume, Chapter 8) and to explore the implications of a variety of 
other more gradual and more rapid rates of warming for sea-level rise. 
High priority should also be given to strengthening our understanding 
of the possibilities of disintegration of the WAIS and the rate at 
which this might occur. One promising approach to these questions is 
through examination of the morphology of reef corals at different 
depths in terraces formed during the last interglacial period about 
125,000 years ago, when the ice mass in question may previously have 
disappeared. 



79 

Other studies and monitoring programs should also be undertaken. In 
doing so, five problems deserve special emphasis: possible changes in 
the mass balance of the Antarctic Ice Sheet; interaction between the 
Ross and Filchner-Ronne ice shelves and adjacent ocean waters; 
ice-stream velocities and mass transport into the Amundsen Sea from 
Pine Island and Thwaites Glaciers; modeling of the ice-sheet response 
to CO 2- induced climate change; and deep coring of the West Antarctic 
Ice Sheet to learn whether it did in fact disappear 125,000 years ago. 

1.5.6.7.2 The Arctic Environment 

A number of research efforts should bring progress in understanding 
effects of a greenhouse warming on the Arctic. Specifically, efforts 
should be made to 

1. Improve GCMs and other models (sea ice, Arctic stratus, ocean 
dynamics, radiation balance, for example) and use them in studies 
focused on Arctic response. Proper handling of cloud cover in the 
Arctic merits special attention as do sensitivity studies using 
improved sea-ice models. 

2. Study stability of the Arctic Ocean density stratification and 
the potential for its destruction. 

3. Obtain long central Arctic sediment cores that could improve the 
record of variations in the Arctic Ocean for the period from 10,000 to 
15,000,000 years ago. 

1.5.6.7.3 Agr icultur e 

Basic research on agriculture in relation to CO 2 assessment falls 
into two broad categories: (1) effects of C0 2 on photosynthesis and 
plant growth and (2) predicting the changes in yield that will follow a 
change to a warmer, drier (or other forecast) climate. With respect to 
the first category, research should be pursued in five obviously related 
areas: rate of photosynthesis, duration of photosynthesis, and fate and 
partitioning of photosynthate ; drought and transpiration; relationship 
of increasing C0 2 to demand for and availability of nitrogen and other 
nutrients; phenology; and weeds. With respect to the second category, 
the relationship of climate to agriculture should be explored through 
both historical (e.g., regression) studies and through simulation. A 
difficult area that deserves consideration is the relationship of 
changing climate to insect pests and pathogens. Finally, attention 
should be given to analyzing effects of concurrent changes in climate 
and atmospheric composition on agriculture, as we make progress on the 
individual aspects. While our assessment is that near-term effects of 
CO 2 and climate change on U.S. agriculture will be modest, the 
evaluation needs to be extended to other regions and, if sound methods 
are available, to more distant times. 

1.5.6.7.4 Ecosystem Response 

The response of ecosystems to projected atmospheric conditions 
remains largely unexplored. Research is called for on ecosystem 



80 

character and net ecosystem production in relation to increasing 
atmospheric C0 2 and climatic change. While there is accumulating 
evidence of effects of increasing C0 2 in increasing the growth of 
well-watered, fertili2ed plants, there is a question as to whether 
these effects extend to natural communities. The question arises 
particularly with respect to forests, where plants live in conditions 
of extreme competition for light, water, nutrients, space, and, 
probably, C0 2 during daylight. The question of redistributing 
forests with respect to climate change also needs to be addressed. 
Effects of increasing temperature on respiration of plants have 
received inadequate attention; by comparison, effects of other factors 
on respiration may be small. Finally, distribution of ecosystems over 
the globe induced by C0 2 and climate change may also affect global 
distribution of albedo; this relationship and its feedback to climate 
should be explored. 



1.5.6.7.5 Water Resources 

There is need to develop further the conceptual basis of analysis 
for all river basins; but the relationships between climate and water 
resources are complex and unique to each river basin, so that basin-by- 
basin studies are also needed. Priority should be given to regions 
with large commitments to irrigated agriculture and for basins where 
scanty or overabundant flow is already a problem. The roles of extreme 
events and interannual variability should be kept in mind. More 
specifically research and analysis is needed on 

1. Relations among temperature, rainfall, runoff, and groundwater 
recharge rates; relations during past decadal or longer climatic 
excursions may be indicative of future possibilities. 

2. Regional, seasonal, and interannual characteristics (including 
extremes) of rainfall and evaporation that might occur with a green- 
house warming. * 

3. Societal response to variations in water supply of different 
duration; water management, especially rates of development of 
water-resource systems and institutional change. 

4. Geomorphologic changes during the last 5000 years from the 
perspective of changes in water regime. 

1.5.6.7.6 Human Health 



in t* ea scant ^" 



81 

continuing difficulty of identifying health effects of much less subtle 
changes in the atmosphere, we are skeptical about whether meaningful or 
interpretable results could be available soon in this area and whether 
the research would be cost effective. However, it does deserve further 
consideration. The area of heat stress under climatic conditions and 
the relationship of climate to disease and disease vectors is less 
difficult and also merits attention. 



1.6 CONCLUDING REMARKS 

The C0 2 issue has been with us for over a century, and impacts of 
increasing C0 2 will be experienced in the century to come. Buildup 
of C0 2 in the atmosphere is one of many issues arising from our 
growth in numbers and power on a planet of finite size and limited 
though large resources. Indeed, an argument for continuing attention 
to the C0 2 issue is that it reminds us of the need to solve 
intellectual and societal problems, which are important to solve for 
other, perhaps more immediate, reasons. The skills called for to 
provide better analysis of and response to the C0 2 issue are similar 
to the skills we would like to have to tackle other problems. With 
more insight into the long-term evolution of the economy and 
technology, the carbon cycle, the oceans and the atmosphere and the 
ice, the responses of agriculture and ecosystems to environmental 
change, and why systems and societies collapse or adapt well, and with 
more extensive cooperation among the carbon-rich nations, many other 
problems besides CO 2 might yield to solution. The C0 2 issue has 
proven to be a stimulus to communication across academic disciplines 
and to cooperation among scientists of many nations. While it may be a 
worrisome issue for mankind, it is in some respects a healthy issue for 
science and for people. It is conceivable that C0 2 could serve as a 
stimulus not only for the integration of the sciences but for 
increasingly effective cooperative treatment of world issues. 



REFERENCES 

Augustsson, T., and V. Ramanathan (1977). A radiative-convective model 

study of the CO 2 climate problem. J. Atmos. Sci. 34:448-451. 
Baumgartner, A. (1979). Climatic variability and forestry. In 

Proceedings of the World Climate Conference. World Meteorological 

Organization, Geneva, Switzerland. 
Bentley, C. R. (1983) . The West Antarctic Ice Sheet: diagnosis and 

prognosis. In Carbon Dioxide f Science and Consensus. Proceedings: 

Carbon Dioxide Research Conference, Sept. 19-23, 1983. Dept. of 

Energy CONF-820970, NTIS, Springfield, Va. 
Bolin, B., P. J. Crutzen, P. M. Vitousek, R. G. woodmansee, E. D. 

Goldberg, and R. B. Cook (1983). The Biogeochemical Cycles and 

Their Interactions. SCOPE 24. Wiley, New York. 
Brooks, H. (1982). Science indicators and science priorities. 

Science, Technology, and Human Values 7:14-31. 



82 

Cess, R. D. (1978). Biosphere-albedo feedback and climate modeling. 

J. Atmos. Sci. 35:1765. 
Chan, Y.-H., J. Olson, and W. Emanuel (1980). Land use and energy 

scenarios affecting the global carbon cycle. Environ. Inter nat. 

1:189-206. 
Charney, J. (1979) . Carbon Dioxide and Climate: A Scientific 

Assessment. Report of the Ad Hoc Study Group on Carbon Dioxide and 

Climate, J. Charney, chairman. National Research Council, National 

Academy Press, Washington, D.C. 
Clark, W. C., ed. (1982). Carbon Dioxide Review; 1982. Oxford U. 

Press, New York. 
Clark, W. C., K. H. Cook, G. Marland, A. M. Weinberg, R. M. Rotty, P. 

R. Bell, L. J. Allison, and C. L. Cooper (1982) . The carbon dioxide 

question: a perspective for 1982. In Carbon Dioxide Review; 1982, 

W. C. Clark, ed. Oxford U. Press, New York. 
Donner, L., and V. Ramanathan (1980). Methane and nitrous oxide: 

their effect on the terrestrial climate. J. Atmos. Sci. 37:119-124. 
Edmonds, J. A., and J. M. Reilly (1983). Global energy and CO 2 to 

the year 2050. Institute for Energy Analysis, Oak Ridge, Tenn. 

Submitted to the Energy Journal. 
Elliott, W. P. (1983). A note on the historical industrial production 

of carbon dioxide. Clim. Change 5:141-144. 
Exxon Corporation (1980). World Energy Outlook. Exxon Corp., New 

York, December. 
Fishman, J., V. Ramanathan, P. Crutzen, and S. Liu (1979). 

Tropospheric ozone and climate. Nature 282:818-820. 
Flohn, H. (1982) . Climate change and an ice-free Arctic Ocean. In 

Carbon Dioxide Review; 1982 , W. C. Clark, ed. Oxford U. Press, New 

York, pp. 145-177. 
Gates, W. L., ed. (1979). Report of the JOC Study Conference on 

Climate Models: Performance, Intercomparison and Sensitivity 

Studies, Vols. I and II. GARP Publ. Ser. No. 22. Joint Planning 

Staff, Global Atmospheric Research Programme, World Meteorological 

Organization, Geneva, Switzerland, 1049 pp. 
Gifford, R. M. (1979) . Growth and yield of C0 2 -enriched wheat under 

water-limited conditions. Aust. J. Plant Physiol. 6:367-378. 
Gifford, R. M. , P. M. Bremner, and D. B. Jones (1973). Assessing 

photosynthetic limitation to grain yield in a field crop. Aust. J. 

Agric. Res. 24: 297-307 . 
Glantz, M. H., J. Robinson, and M. E. Krenz (1982). Climate-related 

impact studies: a review of past experiences. In Carbon Dioxide 

Review: 1982. W. C. Clark, ed. Oxford U. Press, New York, pp. 

57-93. 

Hameed, S., R. Cess, and J. Hogan (1980). Response of the global 
climate to changes in atmospheric composition due to fossil fuel 
burning. J. Geophys. Res. 85:7537-7545. 

Hanel, R. A., B. J. Conrath, V. G. Kunde, C. Prabhakara, I. Revah, V. 
V. Salomonson, and G. Wolford (1972). The Nimbus 4 infrared 
spectroscopy experiment, 1. Calibrated thermal emission spectra. 
J. Geophvs. Res. 11:2629-2641. 



83 

Han sen, J., D. Johnson/ A. Lac is, 8. Lebedeff, P. Lee, D. Rind, and G. 

Russell (1981) . Climate impact of increasing atmospheric carbon 

dioxide. Science 213; 957. 
Hardman, L. ., and W. A. Brun (1971). Effect of atmospheric CO 2 

enrichment at different developmental stages on growth and yield 

components of soybeans. Crop. Sci. 11:886-888. 
Hummel, J. R. , and R. A. Reck (1981). The direct thermal effect of 

CHC1F 2 , CH 3 CCl3 and CH 2 C1 2 on atmospheric surface 

temperatures . Atmos. Environ. 15; 379-382 . 
Inter futures Project (1979) . Facing the Future. Organization for 

Economic Cooperation and Development (OECD) , Paris. 
International Institute for Applied Systems Analysis (1981) . Energy in 

a Finite World; A Global Systems Analysis. Ballinger, Cambridge, 

Mass. 
Jensen, M. E. (1982). Overview-irrigation in U.S. arid and semiarid 

lands. Prepared for the Office of Technology Assessment, 

Water-Related Technologies for Sustaining Agriculture in Arid and 

Semiarid Lands. 
Jones, P. D. (1981). Summary of climatic events during 1981. Clim. 

Mon. 10; 113-114. 
Jones, P. D., and T. M. L. Wigley (1980). Northern hemisphere 

temperatures, 1881-1979. Climate Monitor 9; 43-47. 
Keeling, C. D., and R. B. Bacastow (1977). Impact of industrial gases 

on climate. In Energy and Climate. Geophysics Study Committee, 

National Research Council, National Academy Press, Washington, D.C. 
Langbein, W. B., et al. (1949). Annual Runoff in the United States. 

U.S. Geological Survey Circular 5. U.S. Dept. of the Interior, 

Washington, D.C. (reprinted, 1959) . 
Lorenz, E. N. (1970) . Climatic change as a mathematical problem. J_. 

Appl. Meteorol. 9(3) :325-329. 
Luther, F. M., and M. C. MacCracken (1983). Empirical Development of a 

Climate Response Function; An Analysis of the Propositions of 

Sherwood B. Idso. Report UCID-19792 (draft) . Lawrence Livermore 

Laboratory, Livermore, Calif., March 1983, 56 pp. 
Mar, B. (1982). Commentary. In Carbon Dioxide Review; 1982, W* C. 

Clark, ed. Oxford U. Press, New York, pp. 96-98. 
Marchetti, C. (1980). On energy systems in historical perspective. 

International Institute for Applied Systems Analysis, Laxenburg, 

Austria. 
Marchetti, C., and N. Nakicenovic (1979). The dynamics of energy 

systems and the logistic substitution model. RR-79-13. 

International Institute for Applied Systems Analysis, Laxenburg, 

Austria. 
Marland, G., and R. M. Rotty (1983). Carbon dioxide emissions from 

fossil fuels: a procedure for estimation and results for 

1950-1981. DOE/NBB-0036. NTIS, Springfield, Va. 
Matthews, S. W. (1976). What's happening to our climate? Nat. 

Geographic Mag. 150; 5 76-615. 
Mauney, J. R., K. E. Fry, and G. Guinn (1978). Relationship of 

photosynthetic rate to growth and fruiting of cotton, soybean, 

sorghum, and sunflower. Crop Sci. 18; 259-263. 



84 

Mercer, J. H. (1978). West Antarctic Ice Sheet and CO 2 greenhouse 

effect: a threat of disaster. Nature 271:321-325 (January 1978). 
Meyer-Abich (19&0) . Chalk on the white wall: on the transformation of 

climatological facts into political facts. In Climatic Constraints 

and Human Activities, J. Ausubel and A. K. Biswas, eds. Pergamon 

Press, Oxford. 
Mitchell, J. M. (1979) . Some considerations of climatic variability in 

the context of future C0 2 effects on global-scale climate. In 

Workshop on the Global Effects of Carbon Dioxide from Fossil Fuels , 

W. P. Elliott and L. Machta, eds. CONF-770385. U.S. Dept. of 

Energy, Washington, D.C., pp. 91-99. 
Nakicenovic, N. (1979) . Software Package for the Logistic Substitution 

Model. Report RR-79-12. International Institute for Applied 

Systems Analysis, Laxenburg, Austria. 
National Research Council (1975) . Understanding Climatic Change; A 

Program for Action. Report of the U.S. Committee for the Global 

Atmospheric Research Program. National Academy of Sciences, 

Washington, D.C., 239 pp. 
National Research Council (1982) . Carbon Dioxide and Climate; A 

Second Assessment. Report of the C02/Climate Review Panel, J. 

Smagorinsky, chairman. National Academy Press, Washington, D.C. 
Niehaus, F., and J. Williams (1979). Studies of different energy 

strategies in terms of their effects on the atmospheric C0 2 

concentration . J. Geophys. Res. 84; 3123-3129 . 
Nordhaus, W. D. (1977). Strategies for the control of carbon dioxide. 

Cowles Foundation Discussion Paper No. 443. Yale U., New Haven, 

Conn. 

Nordhaus, W. D. (1980). Thinking about carbon dioxide: theoretical 

and empirical aspects of optimal control strategies. Cowles 

Foundation Discussion Paper No. 565. Yale U. , New Haven, Conn. 
Olson, J. S., and J. A. Watts (1982). Carbon in land vegetation. In 

Carbon Dioxide Review; 1982. W. C. Clark, ed. Oxford U. Press, New 

York, pp. 435-437. 
Paltridge, G. W., and C. M. R. Platt (1976). Radiative Processes in 

Meteorology and Climatology. Elsevier, New York, 318 pp. 
Perry, H., and H. H. Landsberg (1977). Projected world energy 

consumption. In Energy and Climate. Geophysics Study Committee, 

National Academy of Sciences, Washington, D.C. 
Ramanathan, V, (1975). Greenhouse effect due to chlorof luorocarbons : 

climatic implications. Science 190:50-52. 
Revelle, R. R. (1982). Carbon dioxide and world climate. Sci. Am. 

247(2) :36-43. 

Revelle, R., and W. Munk (1977). The carbon dioxide cycle and the 

biosphere. In Energy and Climate. Geophysics Study Committee, 

National Research Council, National Academy of Sciences, Washington, 

D.C. 

Rotty, R. (1977) . Present and future production of C0 2 from fossil ( 
fuels. ORAU/IEA(0) -77-15. Institute for Energy Analysis, Oak 
Ridge, Tenn. 



85 

Rotty, R. , and G. Marland (1980). Constraints on fossil fuel use. In 

Interactions of Energy and Climate , W. Bach, J. Pankrath r and J. 

Williams, eds. Reidel, Doredrecht, pp. 191-212. 
Schilling, H. D., and R. Hildebrandt (1977). PrimSrenergie-Elektrische 

Energie. Cluckauf, Essen, FRG. 
Schneider, S. H., and R. E. Dickinson (1974). Climate modeling. Rev. 

Geophys. Space Phys. 12:447-493. 
Sioli, H. (1973) . Recent human activities in the Brazilian Amazon 

region. In Tropical Forest Ecosystems in Africa and South America, 

B. J. Meggers et al., eds. Smithsonian Institution, Washington, D.C. 
Sionit, N., H. Helmers, and B. R. Strain (1980). Growth and yield of 

wheat under CC>2 enrichment and water stress. Crop Sci. 20:687-690. 
Sionit, N., D. A. Mortensen, B. R. Strain, and H. Helmers (1981). 

Growth response of wheat to C02 enrichment and different levels of 

mineral nutrition. Agron. J. 74:1023-1027. 
Stewart, H. (1981) . Transitional Energy Policy 1980-2030. Pergamon, 

Oxford . 
Stockton, C. W., and W. R. Boggess (1979). Geohydrological 

implications of climate change on water resource development. U.S. 

Army Coastal Engineering Research Center, Fort Belvoir, Va. 
Takahashi, T., D. Chipman, and T. Volk (1983). Geographical, seasonal, 

and secular variations of the partial pressure of CO 2 in surface 

waters of the North Atlantic Ocean: the results of the North 

Atlantic TTO Program. In Proceedings; Carbon Dioxide Research 

Conference; Carbon Dioxide, Science and Consensus. U.S. Dept. of 

Energy Report CONF 820970, Part II, pp. 123-145. 
U.S. Committee for the Global Atmospheric Research Program (1975). 

Understanding Climatic Change; A Program for Action. National 

Academy of Sciences, Washington, D.C., 239 pp. 
U.S. Department of Energy (1982). Effects of CO 2 on mammalian 

organisms. Report of a workshop, June 5-6, 1980, Bethesda, 

Maryland. CONF-8006249. NTIS, Springfield, Va. 
U.S. Water Resources Council (1978) . The Nation's Water Resources, The 

Second National Water Assessment. U.S. Govt. Printing Office, 

Washington, D.C. 
Vinnikov, K. Ya., G. V. Gruza, V. F. Zakharov, A. A. Kirillov, N. P. 

Kovyneva, and E. Ya. Ran'kova (1980). Current climatic changes in 

the northern hemispere. Soy. Meteorol. Hydrol. 6:1-10. 
Wang, W. C., and P. H. Stone (1980). Effect of ice-albedo feedback on 

global sensitivity in a one-dimensional radiative-convective climate 

model. J. Atmos. Sci. 37:545. 
Wang, W. C., Y. Yung, A. Lacis, T. Mo, and J. Hansen (1976). 

Greenhouse effects due to man-made perturbation of trace gases. 

Science 194:685-690. 
Wang, W. C., J. P. Pinto, and Y. Yung (1980). Climatic effects due to 

halogenated compounds in the earth's atmosphere. J. Atmos. Sci. 

32:333-338. 
White, G. F. (1983) . Water resource adequacy: illusion and reality. 

Natural Resources Forum 7(1) : 11-21. 



86 

Woodwell, G. M., R. A. Houghton, and N. R. Tempel (1973). Atmospheric 

C02 at Brookhaven, Long Island , New York: patterns of variation 

up to 125 meters. J. Geophy. Res. 78:932-940. 
Wong/ S. C. (1979) . Elevated atmospheric partial pressure of C(>2 and 

plant growth. Oecologia 44:68-74. 
World Climate Programme (1981) . On the assessment of the role of CO 2 

on climate variations and their impact. Report of a WMO/UNEP/ICSU 

meeting of experts in Villach, Austria/ November 1980. World 

Meteorological Organization, Geneva, Switzerland. 
World Energy Conference (1978) . World Energ ' Resources 1985-2020, An 

Appraisal of World Coal Resources and Their Future Availability; 

World Energy Demand (Report to the Conservation Commission) . IPC 

Science and Technology Press, Guildford, U.K. 
World Meteorological Organization (1983) . Report of the Meeting of 

Experts on Potential Climatic Effects of Ozone and Other Minor Trace 

Gases. Report No. 14. WMO Global Ozone Research and Monitoring 

Project, Geneva, Switzerland, 38 pp. 



Future Carbon Dioxide Emissions 
2 from Fossil Fuels 



2.1 FUTURE PATHS OF ENERGY AND CARBON DIOXIDE EMISSIONS 
William D. Nordhaus and Gary W. Yohe 

This section deals with the uncertainty about the buildup of C0 2 in 
the atmosphere. It attempts to provide a simple model of CO 2 
emissions, identify the major uncertain variables or parameters 
influencing these emissions, and then estimate the best guess and 
inherent uncertainty about future C02 emissions and concentrations. 
Section 2.1.1 is a self-contained overview of the method/ model, and 
results. Section 2.1.2 contains a detailed description of sources, 
methods, reservations, and results. 



2.1.1 Overview 

There is widespread agreement that anthropogenic carbon dioxide 
emissions have been rising steadily, primarily driven by the combustion 
of fossil fuels. There is, however, enormous uncertainty about the 
future emission rates and atmospheric concentrations beyond the year 
2000; and even greater uncertainty exists about the extent of climatic 
change and the social and economic impacts of possible future trajec- 
tories of carbon dioxide. Yet, if the appropriate decisions are to be 
made, the balance of future risks and costs must be weighed, and 
producing best possible estimates of future emission trajectories is 
therefore imperative. 

Many of the early analyses of the carbon dioxide problem have 
produced estimates of future emissions and concentrations from extrapo- 
lative techniques (see Section 2.2, the accompanying survey by Ausubel 
and Nordhaus) . For the purposes of understanding future outcomes and 
policy choices, these techniques leave important questions unanswered. 
First, they do not allow an assessment to be made about the degree of 
precision with which the forecast has been constructed. Moreover, 
little information is generated about the underlying structure that 
produced the reported trajectories. It is, therefore, hard to know how 
changing economic structures might alter the pattern of CO2 emissions. 
But information about precision and sensitivity are sometimes of criti- 
cal importance to policy makers. It is critical to know not only the 

87 



88 

best scientific assessment of an event but also the extent to which that 
judgment is precisely or vaguely known. Particularly in cases where 
policy decisions are irreversible, for example, the best decision in 
the face of great uncertainty might be simply to gather more informa- 
tion. But that decision would be extremely difficult to reach without 
some notion of the extent of the uncertainty surrounding the 
projections. 

In an attempt to address uncertainties, a second generation of 
studies, employing scenario analysis, has arisen. These studies, 
notable among them Limits to Growth (Meadows et al., 1972), CONAES 
(Modeling Resource Group, 1978) , and 1 1 AS A (1981) , have traced time 
paths for important variables with a well-defined model and specified 
sets of assumptions. The studies, which we call "nonprobabilistic 
scenario analysis, 11 represent a marked improvement over earlier efforts. 
They still fall short of providing the policymaker with a precise notion 
of the likelihood that a particular combination of events might occur: 
Is the "high" scenario 1 in 10, 1 in 100, or what? 

In this section an effort is made to put more definite likelihoods 
on alternative views of the world. The technique, called probabilistic 
scenario analysis, extends the scenario approach to include modern 
developments in aggregate energy and economic modeling in a simple and 
transparent model of the global economy and carbon dioxide emissions. 
Particular care is given not only to assure that the energy and produc- 
tion sectors are integrated but also to respect the cost and avail- 
ability of fossil fuels. 

In addition, the analysis presented here attempts to recognize the 
intrinsic uncertainty about future economic, energy, and carbon cycle 
developments. This is done by specifying the most important uncertain 
parameters of the model, by examining current knowledge and disagreement 
about these parameters, and then by specifying a range of possible out- 
comes for each uncertain variable or parameter. The emphasis is not to 
resolve uncertainties but to represent current uncertainties as accu- 
rately as possible and integrate them into the structure in a consistent 
fashion. The result of the entire process is the generation of a range 
of paths and uncertainties for major economic, energy, and carbon 
dioxide variablesprojections of not only a "best guess" of the future 
paths of important variables but also a set of alternative trajectories 
and associated probabilities that quantify the range of possible out- 
comes on the basis of the current state of knowledge about the under- 
lying uncertainty of the parameters. 

It is reasonable, even at this early stage, to ask why such an 
elaborate effort to quantify uncertainty should be undertaken. Cannot 
prudent policy be written on the basis of the "best-guess" trajectories 
of important variables? In general, the answer is "no." To limit 
analysis to the best-guess path is to limit one's options. Similarly, 
to consider some possible path with no assessment of its likelihood 
relative to other possible paths is a formula for frustration, leading 
to endless arguments about which path should be taken most seriously. 
But to consider a full range of possibilities, along with each one's 
likelihood, allows a balanced weighing of the important and the 
unimportant in whatever way seems appropriate. 



89 

Armed with probabilistic scenarios, in other words, the policymaker 
will be able to evaluate a new dimension of his problem. He can assess 
not only a policy along a most likely trajectory but also along other 
trajectories that cannot be ruled out with some degree of statistical 
significance. With some knowledge of the range of uncertainty, he might 
decide to ask for more information to narrow the range, particularly if 
a policy seems to be warranted only by a few selected outcomes. Alter- 
natively, he might choose to minimize the risk of proceeding along an 
undesirable set of possible paths. And finally, he might undertake a 
policy based simply on an expected value. Any one of these options 
might prove to be prudent, but none of them is possible without a quan- 
tified range of possibilities. It is toward providing such a range 
that this section is directed. 

The plan of the overview is this. We first sketch the model that is 
used to relate the different variables and project future carbon dioxide 
emissions. We then describe the data sources and some adjustments that 
we have made to the data. Finally, we describe the results. It should 
be noted that a full description of the methods is contained in Section 
2.2. 

The economic and energy model is a highly aggregative model of the 
world economy and energy sector. It is based on the idea of a multi- 
input production function that represents the relationship between 
world Gross National Product (GNP) (the output) , on the one hand, and 
labor, fossil fuels, and nonfossil fuels (the inputs), on the other. 
In addition, to reflect the likelihood that economic efficiency will 
continue to improve in the future, various technological parameters are 
included to describe the rate of growth of economic efficiency in 
general, as well as the extent to which that growth is more or less 
rapid in the energy sectors than in the nonenergy sectors. 

A further important feature is the explicit incorporation of both 
the extent to which it is relatively easy or difficult to substitute 
nonenergy inputs (insulation or radial tires) for energy inputs (heat- 
ing oil or gasoline) and the extent to which it is easy or difficult to 
substitute nonfossil energy (nuclear or solar-derived electricity or 
hydrogen) for fossil energy (coal-fired electricity or gasoline) . 

The prices of different inputs play a central role in reflecting 
scarcity and driving the relative quantities of different inputs. We 
thus introduce a cost function for fossil fuels that relates their price 
to their degree of exhaustion or abundance. On the other side of the 
market we generate an economically consistent derived demand for energy 
from the structure of the production function as the response of eco- 
nomic agents to changes in relative prices of different fuels and other 
inputs. Thus, if fossil fuels are scarce and costly, the system will 
economize on this input and use relatively more nonfossil fuels and 
labor inputs. 

Finally, we recognize that there are a number of important uncer- 
tainties about the model and future trends. We thus incorporate 10 key 
uncertainties in the model. These relate to variables such as the rate 
of population growth, the availability and cost of fossil fuels, the 



90 



rate of growth of productivity/ the extent to which productivity growth 
will be relatively more rapid in fossil fuels versus nonfossil fuels 
or in energy versus nonenergy/ and so forth. A complete list of the 
uncertainties/ with their relative importance in determining the total 
uncertainty is presented later in this overview. 

The data are gathered from diverse sources and are of quite different 
levels of precision. In general/ we have surveyed the recent literature 
on energy and economic modeling to determine what are commonly held 
views of such variables as future population growth or productivity 
growth. Other variables/ such as the ease of substitution or differ- 
ential productivity growth/ were ones that are not the subject of 
common discourse; for these, we examined recent trends or results 
generated by disaggregated studies. 

A much more difficult data problem arose from the need to estimate 
the uncertainty about key parameters or variables. Our starting assump- 
tion was to view the dispersion in results of published studies as a 
reflection of the underlying uncertainty about the variable studied 
This starting point was modified in two respects. First/ it is com- 
monly observed that even trained analysts tend to cluster together 
excessively that is/ they tend to underestimate the degree of uncer- 
tainty about their estimates. To account for this tendency to move 
toward the current consensus/ we have spread out some of the distribu- 
^ S J observations ** slightly less than 50%. Second/ we have also 
imposed our own judgments about the uncertainties in those cases where 
so ?i nal ? ta Or . disa 9 ement existed or where the disagreement was 
so small as to convince us that the predictions were not independent 
We must emphasize that these judgments about the uncertainties in our 

! 

the discretized variahi^ * , a way that the variance of ? 

^^IciDJ.e Was f^fm^ 1 ^^"\ tv% - 

variable. We thus ended up with 3" /-eg o^T^ the conti us ; 

outcomes. Rather than do a c i * ( ~ 59f049) . Afferent possible 
100 or 1000 of the different \S?M Ascription, we settled on sampling 
below, then, should be interpSted UtC mes - The r **ults "ported 
tributions (although the sampling sam Pl e s of the underlying dis- : 

We now turn to brief descriptio^o?^ 6 *" tO be quite Iow) ' 
study with the promise that amorTnol^! "?" f eSUlts of this 



91 



Percentile Doubling Time 

5 After 2100 

25 2100 

50 2065 

75 2050 

95 2035 



*More precisely , if Xj were the value assumed by run j, whose 
underlying sample of the 10 random variables gave it a probability of 

1000 
?jf then the central estimate would be I p j x j- 



taken as the sample mean of 1000 runs.* Carbon dioxide emissions are r 

pro 3 ected to rise modestly to the end of our time horizon, the year 

2100. We estimate that carbon dioxide emissions will grow at about I 

1.6% annually to 2025 , then slow to slightly under 1% annually after > 

2025. Atmospheric concentrations in the average case are expected to I 

hit the nominal doubling level (600 ppm) around the year 2070 : 

These results show a considerably slower emissions rate and carbon 

dioxide buildup than many of the earlier studies (see Ausubel and * 

Nordhaus, Section 2.2) for two major reasons. First, the expected ; 

growth of the global economy is now thought to be slower than had 

earlier been generally assumed; our work includes this new expectation. ! 

Perhaps more importantly, we also include the tendency to substitute * 

nonfossil for fossil fuels as a result of the increasing relative f 

prices of fossil fuels. This is an important effect that has IH 

frequently been ignored. || 

The next result concerns our attention on the degree of uncertainty f'j 

about future carbon dioxide emissions and concentrations. The range of *i 

uncertainty is shown in Figures 2.1 to 2.4. Figures 2.1 and 2.2 show (1 

100 randomly chosen outcomes for carbon dioxide emissions and concentra- f 1 

tions. These are shown mainly to give a visual impression of the range f < 

of outcomes. Figures 2.3 and 2.4 present the five runs that represent || 

the 5th, 25th, 50th, 75th, and 95th percentiles of outcomes where we I < 

measure the outcomes in terms of the cumulative carbon dioxide emissions * 

fc>y the year 2050. It should be emphasized that the percentiles are 1 

derived from the actual sampling distribution. They reflect, therefore, 

"the distribution of outcomes derived from the interaction of expert jj 

opinion on the underlying random parameters and the economic model; ;j ! ' 

they reflect judgment not an objectively derived distribution. | 

Perhaps the most useful graph to study is Figure 2.4, which shows ! 

the percentiles of carbon dioxide concentrations. For a quarter or ? 

tialf century, the inertia built into the economy and the carbon cycle 
leave an impression of relative certainty about the outcomes. After 
the early part of the next century, however, the degree of uncertainty 
t>ecomes extremely large. In terms of our conventional doubling time, 
note the time at which carbon dioxide concentrations are assumed to hit 
600 ppm: 



ii 



92 



100.0H 



? IO.OH 

I 

s 



o 



LU 

z 
o 

00 
DC 

3 ,OH 



0.1- 




117.5 



19.3 



0.4 



1975 



2000 



2025 



2050 



2075 



2100 



YEAR 



FIGURE 2.1 Carbon dioxide emissions for 100 randomly drawn runs 
(billions of tons of carbon per year) . Outcomes of 100 randomly chosen 
runs; the numbers on the right-hand side indicate the mean projected 
yearly emission for the year 2100 and the extreme high and low outcomes. 




2100 



94 



100.0H 



54.9 



o 

to 

O) 

is? 

O 

CO 
CO 

2 

LU 

z 
o 

GO 
CC 

O 



10.0- 




1975 



2000 



2025 
YEAR 



2050 



2075 



2100 



FIGURE 2.3 Carbon dioxide emissions (gigatons of carbon) from a sample 
of 100 randomly chosen runs. The 5th, 25th , 50th, 75th, and 95th 
percentile runs for yearly emissions, with emissions for years 2100, 
2025, 2050, and 2100 indicated. 



From this result, we make the central conclusion: Given current 
knowledge, we find that the odds are even whether the doubling of 
carbon dioxide will occur in the period 2050-2100 or outside that 
period. We further find that it is a l-in-4 possibility that CO? 
doubling will occur before 2050, and a l-in-20 possibility that 
doubling will occur before 2035. 

The next issue addressed is the question of the relative importance 
of different uncertainties. We have computed by two different 
techniques the relative importance of the ten uncertain variables 
discussed above, and the results are shown in Table 2.1. This table 
calculates the contribution to the overall uncertainty that is made by 
each variable taken by itself. 

In one case, shown in column (2) , the contribution is calculated as 
the uncertainty introduced when a variable takes its full range of 
uncertainty and all other variables are set equal to their most likely 



L 



95 



2500.0H 



I 



z 

2 1250.0 



111 
O 

8 



DC 
LU 

a. 

8 

I 



625.0- 



31 2.5 -V 



95th 




1440 



910 
770 



580 
540 



5th 



1975 



2000 



2025 
YEAR 



2050 



2075 



2100 



FIGURE 2.4 Atmospheric concentration of carbon dioxide (5th, 25th, 
50th, 75th, and 95th percentiles; parts per million). The indicated 
percentile runs for concentrations; the numbers on the right-hand side 
indicate concentrations in the year 2100 for each run. 



values. In the first column, the uncertainty is calculated from the 
run in which the full panoply of uncertainties is deployed that is, 
when all other nine uncertain variables are allowed to take their full 
ranges of outcomes. In both cases, we have created index numbers with 
the variable that induces the most uncertainty set equal to 100, and 
other variables are scaled by their ratio of uncertainty added to that 
of the variable with the largest contribution. 

The two indices are used because they convey different information. 
The variable in column (1) is more relevant to uncertainty reduction in 
the real world (because other variables do indeed have uncertainty) j 
but the calculations in column (1) are dependent on the uncertainties 
assumed for the whole range of variables. The numbers in column (2) 
are, for that reason, more robust to misspecification in the uncertainty 
of other variables. 



96 

TABLE 2.1 Indices of Sensitivity of Atmospheric Concentration in 2100 
to Uncertainty about Key Parameters (100 = Level of Effect of Most 
Important Variable^) 



(1) (2) 

Marginal Marginal Variance 

Variance from from Most 

Parameter- Full Sample- Likely Outcome- 



Ease of substitution between fossil 






and nonfossil fuels [l/(r - 1)] 


100 


100 


General productivity growth [A(t)] 


76 


79 


Ease of substitution between energy 






and labor [l/(q - 1)] 


56 


70 


Extraction costs for fossil fuels [g]_] 


50 


56 


Trends in real costs of producing 






energy [h^(t)] 


48 


73 


Airborne fraction for C0 2 emission 






[AP(a)l 


44 


62 


Fuel mix among fossil fuels [Z(t)] 


31 


24 


Population growth [L(t)] 


22 


36 


Trends in relative costs of fossil 






and nonfossil fuels [h 2 (t)l 


(3) 


21 


Total resources of fossil fuels [R] 


(50)- 


5 



lvalue of sensitivity is scaled at 100 for the variable that has the 
highest marginal variance. 

^Notation in the square brackets refers to variable notation in the 
model presented formally in Section 2.2. 

-"Marginal variance" from full sample equals (1) the variance in the 
base case (i.e., with all variables varying according to their full 
range of uncertainty) minus (2) the variance with listed variable set 
at its most likely value (but all nine other variables varying 
according to their full uncertainty). Note that no resampling occurs. 
-Marginal variance from most likely outcome" calculated as the 
variance when the listed variable assumes its full range of uncertainty 
and all nine other variables are set equal to their most likely value. 
^Parentheses indicate that the marginal variance is negative. 

The ranking of the importance of uncertainties shown in Table 2.1 
contains several surprises. First, note that an unfamiliar production 
parameter ranks at the top of both columns the ease of substitution 
between fossil and nonfossil fuels. While some studies have included 
substitution parameters in their model specifications (notably Edmonds 
and Reilly, 1983) , the sensitivity of concentration projections to 
assumptions about substitution has not been noted earlier. A second 
set of variables on the list include those that have been exhaustively 
discussed in the carbon dioxide literature the world resources of 



97 

fossil fuels and the carbon cycle ("airborne fraction") . Our estimates 
indicate that these are of modest significance in the uncertainty about 
future carbon dioxide concentrations. 

Table 2.1 is also extremely suggestive about research priorities in 
the carbon dioxide area. We cannot, it should be emphasized, move 
directly from the source of uncertainties to a budget allocation for 
research funds on carbon dioxide. It may be much easier, for example, 
to resolve uncertainties about the "depletion factor" for carbon fuels 
than about future "productivity growth." As a result research funds 
might therefore be more fruitfully deployed in the first prior area 
than in the second. 

On the other hand, the results suggest that considerably more atten- 
tion should be paid to some uncertainties that arise early in the 
logical chain from combustion to the carbon cycle, particularly better 
global modeling of energy and economy. It is striking, for example, to 
note that the United States supports considerable work on global carbon 
cycle and global general circulation (climate) models, but much less 
attention in the United States has been given to long-run global eco- 
nomic or energy modeling (see Section 2.2 for a further discussion. 

Finally, it is possible to explore more fully the ramifications of 
Figure 2.4 the figure that indicated 5th, 25th, 50th, 75th, and 95th 
percentile trajectories for atmospheric concentrations of carbon 
dioxide. One might ask, what parameters are most influential in 
determining whether the concentration path deviates from the median in 
either direction; Table 2.1 provides the answer. If uncertainty in a 
parameter is significant in its effect on the overall variance of the 
outcome, then it follows that movement in that parameter away from its 
projected median would be significant in its effect on the outcome 
variable. Clearly, therefore, an increase (decrease) in the ease of 
substitution out of fossil fuel as it becomes more expensive would 
significantly increase the likelihood that the concentration trajectory 
would be lower (higher) than the 50th percentile. Similarly, slower 
(more rapid) productivity growth would cause slower growth in energy 
consumption and produce a significant lowering (raising) of the con- 
centration trajectory. 

As a final set of experiments, we have used our procedure to make 
extremely tentative estimates of the effect of energy-sector policies 
that are designed to reduce the burning of fossil fuels. The particular 
policy we investigate is the imposition of fossil fuel taxes, set, for 
illustrative purposes, at $10 per ton of coal equivalent and at a more 
stringent level. Taxes were not chosen for any reason other than 
modeling ease. Any type of emissions restraint can be represented 
analytically by its equivalent tax. These runs use the most likely 
outcome as a base case. Figure 2.5 shows the trajectory of taxes that 
we have investigated, while Figures 2.6 and 2.7 show the effects of the 
different tax policies on the level of carbon dioxide emissions and on 
carbon dioxide concentrations. In general, the taxes lower emissions 
noticeably during the period in which the taxes are in place. The 
effect on concentrations at the end of the twenty-first century of the 
$10 tax are quite modest. These examples are included here only to 
illustrate the nature of a problem that deserves much more attention. 



98 

They suggest, as does some other work in the literature (Edmonds and 
Re illy,- 1983) , that the use of carbon taxes (or their regulatory 
equivalents) will have to be quite forceful to have a marked effect on 
carbon concentrations, even if they are imposed worldwide. Unilateral 
regulations would, of course, have to be substantially more restrictive. 



90-1 



80- 



70- 



60-i 

I 

J2 50 



O> 



30- 



20- 



10- 



A 



l\ 





TIME 
Pulse 1980 
Pulse 2025 
Stringent 
Permanent 1 980 
Permanent 2025 



1975 



2000 



2025 



2050 



2075 



2100 



YEAR 



FIGURE 2.5 Taxation on carbon fuel price (1975 dollars per ton coal 
equivalent). The time tracks of a stringent tax and four alternative 
$10 per ton of coal equivalent taxes; the temporary taxes peak at $20 
to accommodate the model. 



99 






0- 



-1- 



-2- 



-3- 



-4- 



JJ 

z -6 

30 

r 



-7- 



-8- 




Permanent Beginning 
in 1980 

X 




Permanent Beginning 
in 2020 



Temporary Beginning 
in 1980 



Temporary Beginning 
in 2020 




Pulse 1980 
Permanent 1980 
Stringent 
Pulse 2025 
Permanent 2025 



1975 



2000 



2025 



2050 



2075 



2100 



YEAR 

I CURE 2.6 Plot of carbon emission versus time for taxed runs* 
eviation in emissions from the base run for various taxes. 



.1.2 Detailed Description of the Model , Data, and Results 

le preceding section provided an overview of the paper its motivation, 
:s methodology, and its results. This section will provide a more 
)mplete description of the procedures. Throughout, an attempt will be 
ide to refrain from using economic jargon and overly technical Ian- 



100 



0- 
-10-j 
-20- 

- 30 "i 

-40-^ 

i^ 

8 

O ^70- 



o 



QC 
III 

X 



-so- 



-100 



-110 



Temporary 
Beginning 
in 1980 



-120- 




Pulse 1980 
Permanent 1980 
Stringent 
Pulse 2025 
Permanent 2025 



1975 



2000 



2025 



2050 



2075 



2100 



YEAR 

FIGURE 2.7 Effect of carbon taxes on atmospheric concentration (parts 
per million per year) . Deviation of run from base run without carbon 
taxes . 



,guage. Where this is impossible, definitions of terms will be given in 
footnotes. It is hoped that the reader who is unfamiliar with the 
terminology will be able to follow the reasoning of the paper by 
referring to these notes. 



2.1.2.1 The Model 

2.1.2.1.1 Methodological Summary 

Before we launch into a detailed description of the data and methods 
used in the present study, it may be useful to give a brief symbolic 
overview of the events to follow. We start by denoting the variables: 



101 

x t = endogenous variables (those determined within the system) , as 

they unfold over time; 
z t - exogenous variables (those determined outside the system) , as 

they unfold over time; 

k = parameters of the system, assumed to be constant over time; 
G = a functional relation, mapping the exogenous variables and 

parameters into the endogenous variables. 

The present study is concerned with the future evolution of CO2 
emissions and concentrations as the endogenous variables these are the 
x t variables. The key exogenous variables (or z t ) are economic 
11 events , w such as population growth or fossil fuel reserves. Parameters 
that relate the variables (the k's) are ones such as the airborne frac- 
tion or the emissions per unit energy produced. 
We thus write our system symbolically as 

x t = G(z t , k). 

The pages that follow describe how we derive the relational function, 
G; how we estimate future trajectories of the exogenous variables, 
z t ; and how we estimate the parameters, k. The central difficulty 
with studies of this kind is that the system is imperfectly known. We 
are not able to forecast the z t with accuracy; indeed, we may not 
know which are the important exogenous variables. The parameters, 
also, are imperfectly known. 

The technique that follows uses a procedure that we have denoted 
probabilistic scenario analysis. We start with a simple representation 
of the system (the function G) . We then estimate future trajectories 
and subjective probability distributions [denoted by g(.) and h(.)] of 
the exogenous variables and parameters z t and k: 

g(z t ), h(k) (judgmental probability distributions on z t , k) 

These then map through the G function to give us a conditional 
probability distribution f (.) on the variables of concern, the x t : 

G:[g(z t ), h(k)] + f(x t ). 

All of this is, unfortunately, much more easily described than accom- 
plished. The major issues that arise are these: First, the G function 
is not known in advance and may be extremely complex. Second, neither 
the exogenous variables nor the parameters are well known. The scien- 
tific and economic literature can be used to illuminate the "best 
guess" about these variables or parameters, however. Third, the 
judgmental probability distributions are ignored in most of the applied 
scientific literature. Attempting to determine these distributions is 
the hardest part of our task. And finally, there is no established 
methodology for developing probabilistic scenarios. The text that 
follows outlines one attempt to overcome these difficulties. 



102 



An aggregate world production function sets the stage.* We chose 
the simplest conventional form that would allow explicit parameters for 
both the share of GNP devoted to energy and the ease of substituting 
between fossil and nonfossil fuels t : 



d(t) c r ,_%&, .*r. U-awJ/r m 

X(t) = A(t)L(t) aW [bE C (tr + (1 - b)E (t) ] , (1) 

where 

X(t) = world GNP at time t in constant 1975 U.S. dollars; 

L(t) s world population at time t; 

A(t) - level of labor productivity at time t in U.S. dollars of 

output per capita; 

(1 - d) = the proportion of GNP devoted to paying for energy; 
E c (t) consumption of fossil fuels at time t in metric tons of 

coal equivalent; 
E n (t) * consumption of nonfossil fuel at time t in metric tons 

of coal equivalent; 
r a parameter reflecting the ease of substitution between 

E c (t) and E n (t); and 
b ' - a parameter reflecting the relative levels of use of E (0 

and E n (0) . 

Equation (1) is not so mysterious as it might first appear. It assumes 
its peculiar form because of well-established techniques in micro- 
economics. They mandate that if a production process with certain 
conventional properties is to be represented mathematically, then the 
researcher is locked into an equation of the general type exhibited in 
(1) . A slightly simpler form exists (the straight Cobb-Douglas form) 
and could have been employed, but that would have restricted the degree 
of substitution between the two types of energy in an arbitrary and 
unacceptable manner. To preserve desired flexibility in the specifi- 
cation of the ease of substitution, Equation (1) is the simplest option 
available. 

Turning now to a brief discussion of some of those desirable 
properties, it is important to note, first, that Equation (1) displays 
constant returns to scale; i.e., doubling L(t), E c (t), and E n (t) 
from any level necessarily doubles output. As growth proceeds, this 



*A production function is a mathematical representation of a 
production process that employs a variety of inputs in a variety of 
combinations to manufacture some type of output. General production 
functions allow for substitution between any of the inputs in response 
to changes in input prices so that the manufacturer can maximize 
profits. 

tThe estimation of production functions is a well-developed, field 
in economics. There have been numerous surveys, of which that by 
Johnston (1972) is perhaps the most comprehensive. Also see Nerlove 
(1965) for a careful study of the identification and estimation of the 
particular production function that we use, the "Cobb-Douglas 11 version. 



103 

feature guarantees that payments to labor employed and for energy 
Consumed exhaust output. It should be noted, at least in passing f that 
almost all production relationships that spring to mind easily, and 
* linos t all that are used in existing studies, display this constant 
returns-to-scale property. Imposing it on our production schedule did 
lot drive us afield of conventional economic modeling. 

Note as well that Equation (1) aggregates the value of all nonenergy 
Inputs into labor the third factor of production. This aggregation 
> reduces a simplification that allows the model to isolate the potential 
substitution both between the two types of energy and between energy 
md other inputs without being unnecessarily cluttered by an index of 
/hat those inputs might be. No assumptions about constant capital- labor 
ratios are being made, and any increased productivity that might be 
:reated by this secondary substitution is captured in the A(t) param- 
eter. Nonetheless, the substitutions of critical importance in an 
energy-emissions model substitution between the two major types of 
energy and substitution between energy and other inputs are both 
.identifiable and quantifiable. We will return to them shortly. 

Before doing so, however, it is convenient to discuss the derived 
Lemand for energy implicit in the functional form of Equation (1) as it 
stands. Since energy demand is derived entirely from production, 
kjuation (1) imposes some unavoidable structure on the demand for 
:nergy. The share of X(t) devoted to paying for energy at any point in 
.iine is, first of all, fixed; i.e., letting P n (t) represent the real 
>rrice of E(t) at time t in 1975 U.S. dollars and P c (t) represent 
.lie real price of E c (t) at time t, then the share devoted to energy 
an be expressed as 



P n (t)E n (t) + P c (t)E c (t) 



- d)X(t) 



(2a) 



or all t. Letting 

E n (t) + E c (t) - E(t) 
epresent total energy demand and 

+ [P n (t)E n (t)/E(t)] * P(t) 



epresent the weighted aggregate price of energy, Equation (2a) can 
iranediately be rewritten in the more convenient form: 



E(t) * (1 - d)X(t)/P(t). 



{2b) 



t becomes clear, therefore, that Equation (1) imposes unitary price 
nd income elasticities on the aggregate energy demand equation. 



price elasticity of demand is a reflection of 
ess of the quantity demanded to changes in the price . 
efined as tte ratio between the percentage change in the quantity 

(continued overleaf) 



104 



Additionally, the particular nested form of Equation (1) necessarily 
requires that the relative demand for E c (t) and E n (t) take the form 



(1 - b) P^(t) 



1/r - 1 



. (3) 

E n (t) " 

Implicit in Equation (1) , then, is the condition that a 1% increase in 
the energy price ratio of fossil to nonfossil fuel must always generate 
a[(r - 1) -1 ]% reduction in the ratio of fossil to nonfossil fuel use. 
Since the parameter r could be arbitrarily specified, however, Equation 
(3) is not nearly so restrictive as Equation (2b) . Still, the point is 
this: by specifying the production function, we fully specify the 
underlying structure of demand for all three of our inputs labor, 
fossil fuel, and nonfossil fuel. 

Quantification of the ability to substitute between the two types of 
energy and between labor and energy now follows straightforwardly from 
the derived demand schedules just noted. To that end, let 

m d ln[E C (t)/E n (t)] 
d ln[P(t)/P n (t)] 

represent the notion of the "elasticity of substitution" between the 
two types of energy; i.e., let s represent a measure of how responsive 
the ratio of fossil to nonfossil fuel consumed worldwide is to changes 
in the relative prices of the two fuels. Logarithmic differentiation 
of Equation (3) then reveals that 

s = (r - I)" 1 . 

If r were to equal 0.5, therefore, s would equal -2.0, indicating 
that any 1% increase in the relative price of carbon-based fuel would 



( continued from overleaf) 

demanded and the percentage change in the price that caused demand to 

change; i.e., 

(price elasticity) = d ln[E(t)]/d ln[P(t)]. 

Given Equation (2b) , therefore, it is clear that the price elasticity 
of the derived demand for energy is (-1) . The income elasticity of 
demand is similarly defined as the ratio between the percentage change 
in the quantity demanded and the percentage change in income. Since 
notationally the income elasticity of demand is d ln[E(t)]/d ln[X(t)], 
it is equally clear that this elasticity must also equal 1 for the 
schedule listed in Equation (2b) . In conclusion, therefore, the 
structure of the production schedule recorded in Equation (1) implies 
that (i) a 1% increase in the price of energy would always cause a 1% 
reduction in the demand for energy, while (ii) a 1% increase in world 
GNP would always cause a 1% increase in the demand for energy. 



105 

produce a 2% reduction in the carbon to noncarbon fuel consumption 

ratio. A similar computation meanwhile shows that the corresponding 

elasticity of substitution between either E c (t) or E n (t) and 

labor is unity. Thus, the unitary price and income elasticities of 

aggregate energy demand already noted from Equation (2b) were to be 

expected. 

The lack of flexibility in this last elasticity was a source of 
concern. We were not anxious to be boxed into a structure of unitary 
elasticities in the demand for energy, but we were bound by a well-known 
result of economic theory: in maintaining the simple production struc- 
ture that we felt was required to preserve the necessary transparency 
in the inter temporal model, we were forced to set the elasticity of 
substitution between energy and labor either to s (r - 1)""^ or to 
unity. Rather than loosen this theoretical binding by resorting to a 
more complicated production function, we chose instead to provide the 
desired flexibility by keeping Equation (1) as our fundamental produc- 
tion relationship and adjusting the share of world GNP used to pay for 
energy over time. To see how this was accomplished, let 

a 1/q 
X(t) = A(t) [mL(t) q + (1 - m)E(t) q ] (I 1 ) 

represent the next logical generalization of production. The parameter 
q here reflects the ease of substituting between energy and labor in 
the production process; it is the analog to the parameter r in Equation 
(1), and (q - I)"" 1 is the corresponding elasticity between energy now 
aggregated into one factor and labor. The resulting derived demands 
for labor and energy could then be combined to form the analog of 
Equation (3) : 

ECtl d-nO 



L(t) m w(t) ' 

where w(t) represents the unit price of labor. Multiplying both sides 
of Equation (4) by [P(t)/w(t)], a more convenient form emerges: 

II - d(t) . m P(t)E(t) m kp(t) q/(q - 1) (4I) 

d(t) w(t)L(t) kP(t) ' (4 } 



where k [ (1 - nO/m] " and taking the approximation that w{t) 
!* Notice that the first part of that equation simply states that 
the relative share of GNP devoted to paying for energy must equal the 
ratio of the energy bill of the world, P(t)E(t), and the global wage 
bill w(t)L(t). It makes the d(t) parameter defined here the precise 



* Sett ing w(t) = 1 requires an approximation, as follows: 
The model assumes that if the relative price of energy to labor, 
[P(t)/w(t>], is constant, then d (the share of labor) is constant. We 
are attempting to examine the effect of changes in P(t)/w(t) on d. 

(continued overleaf) 



106 

analog of the d parameter recorded in Equation (1) . Rearranging 
terms, then, the equation 

d(t) * [kP(t)q/(q - 1) + I]" 1 (5) 

provides a means by which the share of world GNP paid to energy can be 
adjusted from period to period in a manner consistent with an elasticity 
of substitution between labor and energy equal to (q - I)" 1 * We were 
able, with this procedure, to approximate a more general schedule like 
Equation (I 1 ) with a series of simpler schedules of the type shown in 
Equation (1) by simply adjusting energy's share of GNP in a way that 
was consistent with the more complex structure that we needed. We are, 
in other words, out of our theoretical bind. 

With this final step completed, we are able to set both the elas- 
ticity of substitution between fossil and nonfossil fuels and the 
elasticity of substitution between energy and labor equal to whatever 
the data suggested were appropriate values without overburdening the 
model with unnecessary complication. 

To proceed from this point it is necessary to specify how A(t), 
L(t), and the prices of the two fuels are to be determined over time. 
Productivity and population are the easiest; they take the forms 

A(t) 
and 

L(t) 

where 

A Q = labor productivity at t = (1975) ? 

a(t) rate of growth of labor productivity at time t; 

LQ world population at t = 0; and 

l(t) - rate of growth of population at time t; 

The last three are taken to be exogenous. 



( continued from overleaf) 

If we were instead to set w(t) equal to its model solution, then 
Equation (4 1 ) could be written as 

A(t) = ke (t) 7 , ( 4") 

whete X - (1 - d)/d, e - P/w, 7 = q/(l - q) . Thus the change in X from 
one path to another would be 

Aln X(t) - y[AlnP(t) -Alnw(t)]. 

For given L(t) and P(t), we can solve this for w(t), and A In w(t) is 
an order of magnitude smaller than Aln P(t), because the share of E is 
about one tenth of the share of L. Our approximation thus misstates 
the change in d by about one tenth. Also note that the share of L 
changes only a little. 



107 

Energy prices are divided into production and distribution cost 
components (roughly the difference between wholesale and retail 
prices) , and production costs are presumed to be subject to 
technological change. The price of noncarbon-based fuel is, more 
specifically, given by 

P n (t) =P^ + pJe [h l (t) + V t)lfc , (6a) 

where 

p d = distribution costs in 1975 U.S. dollars per metric ton 
of coal equivalent; 

PQ = initial production costs in 1975 dollars per metric ton of 
coal; 

-t^tt) a rate of technological change in the energy industry at 
time t; and 

-h 2 (t) - bias of technological change toward noncarbon energy at 
time t 

are all exogenous. The last two entries in the list may require a 
little explanation. The rate of technological change in the energy 
industry is the rate at which the efficiency in the industry is improv- 
ing; conversely, it can be viewed as the inverse of the rate of change 
in the real price of energy. If, for example, the price of energy were 
decreasing at a rate of 1% per year, this would be consistent only with 
a rate of technological change of 1% per year. The bias in the rate of 
technological change reflects the possibility that technical change and 
innovation will not proceed at the same rate in both energy sectors. If 
innovation were more rapid in the nonfossil fuel sector, for instance, 
then the bias would favor that sector, and h 2 (t) would be positive. 
The price equation for fossil fuel takes similar form. The only 
complicating element here is the implicit inclusion of a depletion 
factor a reflection of the usual expectation that the price of fossil 
fuel should increase over time as the world's resources of fossil fuels 
are used up. We do not, here, necessarily include supply and demand 
effects.* The depletion factor represents, more accurately, the notion 



*The model has one theoretical flaw from the point of view of the 
economics of exhaustible resources. This is that there are no rents 
charged to scarce fossil fuels. The economics and some estimates of 
such scarcity rents are provided in Nordhaus (1979) for a model without 
uncertainty. 

There is a very great difficulty in the present model, however, in 
calculating the appropriate scarcity rents. This difficulty arises 
because the appropriate scarcity rents will be different in each of the 
3 10 possible trajectories. And the actual rent at each point of time 
will depend on the way that the uncertainties are revealed over time. 

(continued overleaf) 




108 

that the price of fossil fuel must increase as the cheaper wells are 
drilled or mines are exhausted and as more expensive sources come on 
line. Depletion is represented as follows: 

f(t)t 
9n + 9i { R (t)/[R - R(t)]P)e 1 + T(t - t), (6b) 

where 

c 
P - distribution costs in 1975 U.S. dollars per metric 

ton of coal equivalent; 
g = initial production costs in 1975 dollars per metric 

ton of coal equivalent; 
R = a measure of the world's remaining carbon-based fuel 

reserves in metric tons of coal equivalent in 1975; 
T(t - t) a tax policy parameter used to reflect taxation of fossil 

fuels; 
R(t) [E c (0) + ... + E c (t - 1)] total carbon-based fuel 

consumed since 1975 in metric tons of coal equivalent; and 
g.j(i 1,2) = depletion parameters. 

In this list, of course, all but R(t) are taken to be exogenous. 

At this point, then, only three parameters remain to be determined: 
b, A, and m. These are specified, given assumed values for s, r, q, d, 
L Q , E c (0), E n (0), (p]J + P*), and (g Q + P^) , so that the entire set of 
parameters satisfied Equations (1) , (3) , and (5) at time zero. No 
further data are necessary. 

Special mention needs to be made of the policy variable T(t - t) . 
It is included to reflect any policy that might be designed to reduce 
carbon dioxide concentrations either directly by taxes or indirectly by 
discouraging the consumption of fossil fuels. Since either type of 
policy would make it more expensive to burn these fuels, either would 
be captured by the tax T(t - t) that increased the price of E c (t) . 
The parameter t simply denotes the lag between the imposition of a 
C(>2 reduction policy and its effect on the day-to-day operations of 
fuel burners. The use of a tax to summarize even quantity-based 
restrictions is widespread in the economic literature and is supported 
by the following equivalence theorem: for any targeted quantity 
restriction on, for example, carbon-based fuels, there exists a tax to 



(continued from overleaf) 

After some thought about the best way to calculate the rents, we 

finally gave it up as hopelessly complicated. 

In reality, it seems that, except for oil and gas, the scarcity 
rents are likely to be quite small for most of the time. This 
conclusion is based on a reading of the estimates from Nordhaus 
(1979). However, it should be noted that omission of the scarcity rent 
leads to a downward bias in the market price of fossil fuels and 
consequently in an upward bias in the estimate of C0 2 emissions and 
concentrations. We expect that this bias is likely to be on the order 
of to 2% during the period under consideration. 



109 

be added to the price of carbon-based fuels such that consumers, in 
their own best interest, will undertake actions to lower their 
consumption to the prescribed target level.* Either tool, properly 
computed, can therefore achieve any arbitrary policy objective, and the 
generality of the tax approach is assured* 

Some have noted that the two alternatives need not be equivalent, in 
terms of their efficiency, under uncertainty* A similar theorem exists, 
however, when the comparison is conducted between alternatives computed 
to generate the same expected result* Others have worried that the 
equivalent tax might, in practice, be difficult to compute. Whether 
that is true or not, of course, this purported difficulty does not 
damage the treatment in the present paper. 

Returning now to the model, Equations (1) and (6) complete a simple, 
economically consistent vehicle with which to project the driving force 
of industrial CO 2 emissions. Only a link to the atmosphere is 
required; that link is represented by 

C(t) 

where 

C(t) = emissions of carbon in gigatons per year; 

ZQ * the "emissions factor," equal to the initial ratio of carbon 

emissions to fossil fuel consumption in 1975; and 
z(t) = the rate of growth of the emissions factor. 

The last two are, of course, exogenous. The ratio z(t) is, moreover, 
presumed to increase over time because of a supply- induced change in 
the fuel mix (i.e., toward coal and shales). Nonfossil fuels are 
presumed to provide energy without adding to carbon emissions. 

An airborne fraction approach to link emissions to atmospheric 
concentrations is finally employed to complete the model. Formally, 

M(t) = M(t - 1) + AF(s) [0.471 C(t)] - sM(t - 1), (7) 

where 

s a seepage factor reflecting the slow absorption of airborne 

carbon dioxide into the deep oceans; 

AF(s) = the marginal airborne fraction of carbon dioxide; and 
M(t) carbon mass in the atmosphere in period t measured in 

parts per million. 



*See Yohe (1979) for a survey of the literature on this point. 



110 

Equation (7) is a standard representation of the complex workings of 
the atmosphere, frequently used in the carbon cycle literature.* The 
coefficient 0.471 preceding C(t) simply converts gigatons of carbon 
into the appropriate atmospheric units of parts per million. The 
seepage factor is a subject of current debate among researchers (see 
Brewer, this volume. Chapter 3, Section 3.2); in separate work, we have 
shown that the maximum likelihood estimate of the airborne fraction is 
quite sensitive to the specification of s; thus, the AP(s) notation. 

In summary, then, the model operates with the demands for fossil and 
nonfossil fuel being derived entirely from a production function that, 
for any year, assumes the form 



X(t) * A(t)L(t) d(t) [bE C (t) r 
They emerge summarized by 

P n (t)E n (t) + P c (t)E c (t) - [ 



and 




(1 - b) P C (t) 



- b)E n (t) r ] 



- d(t)]X(t) 



1/r - 1 



- d(t)]/r 



(1) 



(2a) 



(3) 



with 

d(t) = [kP(t)q/(q - 1) + I]" 1 . 



(5) 



Furthermore, the neutral productivity growth factor and labor growth 
component of Equation (1) are given exogenously by 



A(t) 



A Q e 



a(t)t 



and 



L(t) - L 

respectively. The supply conditions from fossil and nonfossil fuels 
are meanwhile determined by 



*See Bolin (1981) for a complete discussion of the concentration 
model. Our modification of that work specifies a marginal airborne 
fraction the fraction of period t emissions that remain in the ; 
atmosphere on the margin (i.e., in period t) . Whenever the seepage 
factor is nonzero, the marginal fraction does not equal the average 
fraction that most of the previous studies have employed. 



Ill 



P n (t) - Pj + Pj e [h l (t) + h 2 (t)lt (6.) 




and 



P c (t) = Pfl + lg fl + g,<R(t)/[R - R(t)]^2 J e A + T(t - t) (6b) 
\ .' 

with R(t) = E c (0) + . . . + E c (t - 1) . 

Emissions are then recorded according to 

f I 
C(t) * Zoe 2 ^)* 1 E c (t) 



and atmospheric concentrations according to [ 

M(t) * M(t - 1) + AF(s) [0.471 C(t)] - sM(t - 1). (7) 

Figure 2.8 represents a geometric interpretation of this process. 

2.1.2.2 The Data 

Two kinds of data are required. Initial conditions are, first of all, 
required. Table 2.2 records the estimates for world GNP, world popula- 
tion, and world fossil and nonfossil fuel in 1975. Since these initial 
conditions are based on historical evidence, consensus is not difficult 
to achieve. Existing studies and comparison with published data are 
sufficient to generate consistent estimates for these parameters. 
Initial energy prices are a bit more problematical. We want aggregate 
prices based on the historical distribution of, for example, fossil 
fuels between coal, oil, and gas. Table 2.3 records both the necessary 
raw data and their sources. Table 2.4 produces the aggregates and 
illustrates the procedure that is employed in their construction. Table 
2.5 registers the emissions ratios of the various types of fossil fuels 
from which the initial value for the aggregate emissions ratio is com- 
puted. Table 2.4 also records that aggregation procedure. Finally, an 
initial level of atmospheric carbon concentration is required; current 
measures set the 1975 value at 331 parts per million (ppm) (see Keeling 
et al. in Clark, 1982, Table 1, page 378). 

Data are also required to set the long-term context of the study a 
more difficult problem. Projections of various important parameters 
into the near and distant future were compared, but the uncertainties 
inherent in such projection made consensus impossible* Existing studies 
provide ranges for variables like world population growth, world produc- 
tivity growth, energy prices, and the emissions factor, but no generally 
accepted paths emerge. The observed ranges are, however, viewed as 
more than spurious disagreement among researchers. They are, instead, 
viewed as a reflection of the inherent uncertainty about the variables. 

A more precise description of the technique we use is the following: 
we assume that the published estimates for each of the random variables 



112 




5 






00 


CM 

I 



113 

TABLE 2.2 Initial Conditions for Population, GNP, and Aggregate Fuel 
Consumption 



A. 


World Population in 1975 






IIASA (1981, p. 133) 


4 x 10 9 




Ridker and Watson (1980, p. 45) 


3.8 x 10 9 




Keyfitz (1982) 


3.98 x 10 9 




Value employed: L(0) 


4 x 10 9 


B. 


World GNP in 1975 






IIASA (1981, p. 457) 


$ 6.4 x 10" 




Ridker and Watson (1980, p. 45) 


$ 4.5 x 10 12 




Report of the President (1980) 


$ 6.53 x 10" 




Value employed: X(0) 


$ 6.4 x 10 12 


C. 


Fossil Fuel Consumption in 1975 






IIASA (1981, p. 136) 


8.127 x 10 9 mtce 




Ridker and Watson (1980, p. 185) 


8.114 x 10 9 mtce 




Value employed: E c (0) 


8.1 x 10 9 mtce 


D. 


Nonfossil fuel Consumption in 1975 






IIASA (1981, p. 136) 


0.67 x 10 9 mtce 




Ridker and Watson (1980, p. 185) 


0.69 x 10 9 mtce 




Value employed: E n (0) 


0.7 x 10 9 mtce 



*The Ridker and Watson estimate was ignored because it was built 
around an estimate of per capita income that appeared to be low 
relative to other published data. 

identified in our model is an unbiased, but not necessarily independent, 
estimate of that variable. We use the means and variances of those 
estimates as a basis for constructing judgmental probability distribu- 
tions for each variable. To obtain a manageable number of alternatives 
from which to sample, we assume that each judgmental probability dis- 
tribution is normally distributed. We then take high, middle, and low 
values (corresponding to 25, 50, and 25%, respectively) that maintain 
the same means and variances as the estimated normal distributions. 

To put it more intuitively, we have constructed discrete distribu- 
tions to mirror the level of uncertainty at present surrounding the 
various analysts' published projections. In deference to the tendency 
of individuals to underestimate uncertainty, however, the procedure for 
reflecting uncertainty does not stop there. Particularly when the 
estimated variances declined over time, future variances are expanded 
beyond their computed ranges to correct for systematic underestimation 
of uncertainty. Section 2.1.2.2.1 is devoted to a thorough exploration 
of this second procedural phase? the remainder of this section will 
concentrate on applying the first phase to the critical parameters. 

In passing, though, it should be noted that our procedure produces 
more than a purely subjective view of the future paths of some com- 
plicated variables. It produces a "judgmental" view that weighs the 
expert opinions of many researchers as expressed in their published 



114 
TABLE 2.3 Energy Price and Disaggregate Consumption Data 



A. Consumption Patterns (1975)3. 






I. Fossil Fuel 






Coal 


2.42 x 10 9 mtce 


30% 


Oil 


4.10 x 10 9 mtce 


50% 


Gas 


1.61 x 10 9 mtce 


20% 


Total 


8.13 x 10 9 mtce 


100% 


II. Nonfossil Fuel 






Hydro 


0.53 x 10 9 mtce 


81% 


Nuclear 


0.13 x 10 9 mtce 


19% 


Total 


0.66 x 10 9 mtce 


100% 


B. Energy Prices (1975) 






I. Primary 






Coal 


$ 15/mtce 




Oil 


$ 54/mtce 




Gas 


$ 19/mtce 




Electricity 
II. Secondary^ 


$118/mtce 




Coal 


$ 2 3 /mtce 




Oil 


$ 98/mtce 




Gas 


$ 28/mtce 




Electric ty 


$255/mtce 




C. Energy Prices (1981) 






I. Primary 






Coal 


$ 15/mtce 




Oil 


$108/mtce 




Gas 


$ 86/ratce 




Electric ty 


$118/mtce 




II. Secondary^. 






Coal 


$ 2 3 /mtce 




Oil 


$171/mtce 




Gas 


$116/mtce 




Electric ty 


$255/mtce 





SSource: IIASA (1981), p. 471. 

^Measured in 1975 United States dollars. Source: Reilly et al. 

(1981) . 

fiThese are wholesale prices. 

^These are retail prices. . 

^Measured in 1975 United States dollars. They are derived from the 

1975 prices to reflect the impact of the 1979 Iran-Iraq war as 

follows. The June 10, 1982 f issue of Blue Chip Indicators provided an 

estimate for the 1981 wholesale oil price of $34.00 (current dollars) 

per barrel. That translates into $22.50 per barrel in 1975 dollars, or 

$108/mtce. The btu equivalent price for gas was then computed to equal 

[0.8($108/mtce)] * $96/mtce. Coal and electricity (generated from 

nonfossil fuels) were assumed to remain constant over the 6-year 

period. The secondary prices for oil and gas were then computed by 

adding the reported consensus differences between wholesale and retail 

prices: $13/barrel or $63/mtce for oil and $30/mtce for oil. Thus, 

secondary prices of [$108 + $63] $171/mtce and [$86 + $30] = 

$116/mtce were recorded, respectively. 



L 



115 
TABLE 2.4 Aggregate Prices and Emissions 



A. Energy Prices in 1975.S. 

I. Primary 

Fossil fuelk $ 35/mtce 

Nonfossil fuel- $ 118/mtce 

II. Secondary 

Fossil fuel $ 62/mtce 

Nonfossil fuel $ 255/mtce 

B. Energy Prices in 1981S. 

I. Primary 

Fossil fuel $ 76/mtce 

Nonfossil fuelS. $ 118/mtce 

II. Secondary 

Fossil fuel $ 116/mtce 

Nonfossil fuelS. $ 255/mtce 

C. Emissions Ratio in 1975-: 2(0) 580 g of C/mtce 



^Measured in constant 1975 dollars. 

^Computed using the prices and weights recorded in Table 2.3. In 

1975 r for example/ coal amounted to 30% of the total fossil fuel 

consumed and cost $15/mtce f oil amounted to 50% of the tptal and cost 

$54/mtce, and gas amounted to 20% at a cost of $19/mtce. Thus, the 

aggregate price of fossil fuel in 1975 was 0.3 ($15) + 0.5 ($54) + 

0.2 ($19) $35/mtce. 

The price of nonfossil fuel was taken to be the price of electricity 

generated from nonfossil sources. 

^Computed using the weights and prices recorded in Table 2.3. The 

weights employed were the 1975 numbers because it was unlikely that 

major substitutions could have occurred in the 6 years from 1975 and 

1981. This presumption is borne out by data published in the BP 

Statistical Review of the World Oil Industry , 1980 , p. 16. 

SThe ratio of grams of carbon emitted per mtce of fuel consumed. The 

1975 consumption weights of Table 2.3 were combined with the emission 

data of Table 2.5 to produce Z(0); i.e., Z(0) - 0.3(700) + 0.5(577) + 

0.2(404) = 580 g of C/mtce. 



work. The data for obtaining parameter estimates are not, in other 
words, anonymous and private. They are public and thus presumably 
derived with the care that scientists use in producing work attached to 
their names. And they are judgmental views about nonelemental 
variables not variables like GNP growth or energy growth that depend 



116 
TABLE 2.5 Carbon Emissions from Fossil Fuels* 

Fuel kg of C/10 9 J kg of C/mtcefe 



Petroleum 


19.7 


577 


Gas 


13.8 


404 


Coal 


23.9 


700 


Shale oil 


41.8 


1224 



^Source: Marland (1982). 

-Conversion based on 1 mtce 29.29 x 10 9 J; kg of C, kilograms 

of carbon. 

^Includes carbon dioxide emissions due to shale oil mining and 

extraction. 



on a host of known and unknown effects, but variables like population 
growth and resource availability that depend on fewer things.* 

Beginning once again with population numbers, Table 2.6 shows that a 
variety of growth projections have been made for at least the next 50 
years. Each assumes no major catastrophes; and while the general trend 
in each calls for a steady decline in the rate of growth, there is some 
disagreement. Differences are to be expected, of course, but it is 
interesting to note that these differences found their source in the 
assumptions made about the less-developed countries; the historical 
experience of the LDCs has been so widely varied that a common expecta- 
tion would, of course, have been surprising. The full effect of that 
disagreement is not reflected in the world projections, however, because 
tne larger, more-developed countries have displayed low, stable growth 
rates over the past few decades. 



the matked reduction in the variance of pro- 
* 202 5; ost researchers predict that the world's 
will stabilize sometime after the first third of the twenty- 



*Two technical points might be raised: 

gurefdeDeL^n * Bt ates ^Pendent? It is likely that some of the 
single cfreful .ST^ e f imates -^eed, they might all go back to 

~ 



re 

Second"! tr S Y inde P endent < even competitive. 

indfpSt? whUe^ 1 ^ 9 ^"tal Probability distributions 
care to construct L^ J ingering ^relations probably exist, we took 
lo"-that L the vari^M Var 4 iables so th " the correlations were 
rate of prodictivity Jrowt^ T Int6nded *> ** orth *al. *hus, the 
the difference ilnrJ, ^- 6n6rgy is thought to be independent of 
fuels. Productivity growth between fossil and nonfossil 



L 



117 
TABLE 2.6 Projected Trends in the Growth Rates of World Population^ 

Source of Estimate^ 1975-2000 2000-2025 2025 and beyond 



OECD 


1.6% 




^ tmm 


IIASA 


1.7% 


0.9% 


d 


RFF (high) 


1.9% 


1.52% 


d 


RFF (low) 


1.4% 


0.75% 




Hudson Institute 


2.0% 


1.4% 





KeyfitzS 


1.6% 


0.9% 


0.3% 


Mean 


1.7% 


1.1% 


0.3% 


Standard deviation 


0.2% 


0.36% 


n.a. 


Cell extremes 


1.4%; 2.0% 


0.6%; 1.6% 


n.a. 



^Estimates of the l(t) parameters of population growth equation. 

^Sources: OECD Interfutures Project (1979) , IIASA (1981) , Ridker and 

Watson (1980) f Kahn et al. (1976), and Keyfitz (1982). 

The IIASA projections were based on an earlier set of estimates by 

Keyfitz. 

^The IIASA and RFF studies report the expectation of stable world 

population some time after the first third of the twenty-first century. 

first century. Sometimes they have reached that conclusion because 
they believe that by then the volatile LDC behavior will have evolved 
into the predictable model of the developed countries; sometimes their 
predicted stability was based on some other presumption. In either 
case, their behavioral hypothesis was as much of a guess about the 
unknown future as any other growth path, and it is hard to see why 
uncertainty should diminish as time goes forward. The observed reduc- 
tion in range of population growth projections beyond 2025 is thus a 
likely candidate for the adjustment discussed further in the next 
section. 

As troublesome as the later estimates might have been, however, the 
earlier ranges provided excellent arenas for illustrating the sum- 
marizing procedure for the observed variation. The various estimates, 
ranging from 1.4% growth per year up to 2.0% for 1975-2000 are, for 
example, assumed to be observations drawn from an underlying normal 
distribution of the true uncertainty. These observations (X^) are 
then used to compute estimates of the mean (u x ) and variance 
(a 2 ) of that distribution: 



y x =X "n 
and 



! 





118 

where n represents the number of observations. To discretize the 
distribution defined by X and S 2 into three cells of probabilities 
0.25, 0.50, and 0.25, therefore, X is assigned a probability of 0.5 and 
(X + sS~2) probabilities of 0.25. In this way, mean and variance 
[s 2 ^ 0.25(2s 2 ) + 0.25(2s 2 )] are both preserved. For the 1975-2000 
range, therefore, 1.7% is the "middle" estimate, while 1.4% and 2.0% 
represent the extremes. Under this procedure, roughly 8% of the under- 
lying probability is left beyond the extremes on both sides. Similarly, 
1.0% is the middle estimate for the period 2000-2025, with 0.5% and 1.5% 
catching the 0.25 probability tails. 

Estimates of growth in world productivity are recorded in Table 2.7. 
They are, for the most part, based on a somewhat surprising assumption 
about the growth of world trade over the next several decades. Each 
researcher found that the growth of the world economy will be bounded 
by growth in the largest markets the developed countries. Many studies 
have identified productivity growth as a critical parameter for energy 
and carbon dioxide projections. The common presumption about the 
growth of world trade, ironically, has caused otherwise independent 
studies to project estimates of output growth that converge over time. 
Of particular note is the decline in the variance in projected growth 
rates beyond the year 2025. It may have been caused more by a dearth 
of estimates than anything else, but its range includes only the lower 
tail of the long-run historical experience of the United States, and it 
misses the Japanese experience completely. These ranges, too, are 
subject to revision later. 

TABLE 2.7 Projections of the Rate of Growth of World Productivity^ 



Source of Estimate^ 1975-2000 2000-2025 2025 and beyond 



OECD (high) 


3.4% 








OECD (mid) 


2.8% 


-- 





OECD (mid) 


1.9% 


~ 





OECD (low) 


2.7% 








IIASA (high) 


2.3% 


0.9% 


-- 


IIASA (low) 


1.2% 


1.9% 





Hudson 


2.8% 


1.4% 


1.2% 


RFF (high) 


2.4% 


2.1% 





RFF (low) 


1.6% 


1.8% 





Hudson (low) 








0.75% 


Mean 


2.3% 


1.6% 


1.0% 


Standard deviation 


0.7% 


0.5% 


0.3% 


Cell extremes 


1.2%; 3.4% 


0.9%; 2.3% 


0.5%; 1.5% 



^Estimates of the a(t) parameter in the productivity growth 

expression. 

-Sources: OECD Interfutures Project (i 

Watson (1980) , and Kahn et al. (1976) . 



expression. 

^Sources: OECD Interfutures Project (1979), IIASA (1981), Ridker and 



119 

TABLE 2.8 Projections of the Rate of Growth of Noncarbon Energy 
Prices^ 



Source of Estimate^ 1975-2000 2000-2025 2025 and beyond 



IEA 


0.0% 


0.0% 


0.0% 


RFP (DH NU) 


1.0% 


0.6% 





RFF (DHP1) 


0.7% 


-0.1% 


__ 


RFF (DHP2) 


1.0% 


-0.4% 





Mean 


0.6% 


0.0% 


0.0% 


Standard deviation 


0.5% 


0.4% 


n.a. 


Cell extremes 


-0.1%; 1.3% 


-0.5%; 0.5% 


n.a. 



^Estimates of the h^t) and 1*2 (t) parameters of the energy price 
(supply) equations . 

^Sources: Reilly et al. (1981) , Ridker and Watson (1980). 
The difference between these three scenarios is essentially a 
difference in the assumption about solar and nuclear development. The 
particulars are not so important, for our purpose, as the spread of 
uncertainty. 



Table 2.8 records projected future adjustments in the primary real 
price of noncarbon-based energy electricity not derived from burning 
carbon-based fuel. These trends can, however, be interpreted as the 
inverse of the rate of technological change in the energy sector, i.e., 
in the notation of the previous section, h^(t). Since technological 
change can continue in the fossil fuel sector as well, these estimates 
are also used to frame the difference in the rate of advance between 
the two sectors [h2<t)]. These estimates, then, are clearly dependent 
not only on growth assumptions (and thus the need for new technology) 
but also about the future contributions of sources like nuclear, fusion, 
and solar-generating facilities. Despite the obvious uncertainties 
involved in projecting either factor into the twenty-first century, the 
estimates recorded in Table 2.8 again converge. The Reilly et al. 
(1982) view of constant real prices is therefore included as the middle 
case, and the ultimate variation around that case expanded. 

Estimates of the emissions factor are based both on the unit emis- 
sions for each source recorded in Table 2.5 and on projected mixes of 
carbon-based fuels in the future. For each case, the mix of oil, gas, 
coal, and shale oil is computed and used to weight the unit emissions 
in computing an aggregate. The procedure has already been illustrated 
in the calculation of Z(0) for Table 2.4. Results of the other computa- 
tions are noted in Table 2.9. The summarizing procedure is applied 
across the ranges of emissions for each period to produce high, medium, 
and low trends. Notice that the high trend includes a 31% contribution 
from shales (as projected by RFF) by the year 2050. The lower two paths 
stabilized at 100% coal, or 700 g of C/mtce by 2075. The three pos- 
sibilities are illustrated in Figure 2.9. 



120 



TABLE 2.9 Aggregate Carbon Emissions 



A. Fuel Proportions 
Year SourceS. 


Proportions 


Carbon 
Emissions 


Oil 


Gas 


Coal 


Shale 


1975 Table 2.3 


0.50 


0.20 


0.30 


0.00 


580 


2025 IIASA (high) 


0.33 


0.29 


0.38 


0.00 


580 


IIASA (low) 


0.34 


0.23 


0.43 


0.00 


590 


IEA 


0.28 


0.18 


0.54 


0.00 


612 


RFF 


0.28 


0.23 


0.37 


0.06 


607 


2050 IIASA (high) 


0.25 


0.10 


0.65 


0.00 


604 


IIASA (low) 


0.28 


0.19 


0.53 


0.00 


598 


IEA 


0.26 


0.08 


0.66 


0.00 


644 


RFF 


0.02 


0.02 


0.65 


0.31 


854 


B. Projected Growth in 


Emissions- 


-Z(t) 









Emissions 


Growth 


Year 


Mean 


Std.Dev. Extremes 


Mean 


Extremes 


1995-2025 
2025-205C& 


597 
627 


15 
25 


582; 
602; 


612 
799 






.05% 
.2% 






.0%; 
.1%; 



1 


.1% 
.1% 


2050-20752. 


n.a 


n.a. 


700; 


700; 854 





.4% 





.6%; 





.3% 


2075-21002 


n.a. 


n.a. 


700; 


700; 854 





.0% 





.0%; 





.0% 



^Sources: IIASA (1981) f Reilly et al. (1982), Ridker and Watson 
(1980) . 

The mean and standard deviation reported here excludes the shale 
estimate to generate the low extreme and the middle growth paths, 
higher extreme includes the shale estimate in both computations. 
The two lower runs exclude shale; the high extreme converges to a 
30% shale share of carbon-based fuel. 



The 



Elasticities of substitution between energy and labor, on the one 
hand, and between the two types of energy, on the other, are estimated 
on the basis of the literature on price elasticities of demand. In the 
former case, for example, it is noted that many would put the overall 
price elasticity of demand for energy somewhere in the inelastic range, 
i.e., they would expect a 1% price increase to reduce consumption by 
something less than 1%. A range for s 1 = (q - I)"" 1 , the elasticity 
of substitution between E(t) and L(t), that select -0.4 and -1.2 for 
the 25% probability extremes and -0.7 for the mean is therefore 
employed. Similar reasoning puts the extremes for s = (r - I)" 1 , 
the elasticity between E c (t) and E n (t) , at -0.5 and -2.0 around the 
middle run of -1.2. 



121 



875- 



850- 



825- 



* 800- 



TS 775- 



C 

o 



oc 

V) 



LU 

O 
CO 
DC 



750- 



725- 



700- 



S5 675- 



650' 
625- 



600- 



575-^ 



(854) 




(700) 



550 



. , . , 
2000 



1975 2000 2025 

YEAR 
FIGURE 2.9 Carbon emissions ratio. 



2050 



2075 



2100 



There are by now a wide variety of studies of the elasticity of 
substitution between energy and nonenergy inputs. To a first 
approximation, this parameter is equal to the price elasticity of the 
derived demand for energy [this is shown in Nordhaus (1980a)]. Table 
2.10, drawn from Nordhaus (1980a) , gives central estimates and ranges 
for the price elasticity of the demand for energy. 



122 



TABLE 2.10 Range of Estimated Final Demand Elasticities^ 



Sector 



Hogan 



Nordhaus 



Best Guess 



Implicit 
Elasticity for 
Primary Energy 



Residential 


-0.28 


to 


-1.10 


-0.71 


to 


-1. 


14 


-0 


.9 


-0. 


3 


Transport 
Industrial 


-0.22 
-0.49 


to 
to 


-1.30 
-0.90 


-0.36 
-0.30 


to 
to 


-1. 
-0. 


28 
52 


-0 
-0 


.8 
.7 


-0. 
-0. 


2 
4 


Aggregate 









-0.66 


to 


-1. 


15 


-0 


.8 


-0. 


3 



^Sources: The first column is from Hogan (1980) ; the second column/ 
from Nordhaus (1977); the third column is Nordhaus' s judgmental 
weighting of various studies. To obtain an estimate of the crude price 
elasticity in the fourth column, the final demand price elasticity in 
the third column is divided by the ratio of retail price to crude price. 



For use in the present study we lowered the elasticity to 0.7 
because of some suggestion that price elasticities are lower in less- 
developed countries than in developed countries. In addition, note 
that, while the price elasticities may appear high, they are not for 
two reasons. First, they are long-run rather short-run elasticities. 
And, second, they relate to the elasticity for final energy demand, not 
for primary energy. As is shown in the last column of Table 2.10, 
price elasticities for primary energy are considerably lower than the 
figures we use. 

The elasticity of substitution between carbon- and noncarbon-based 
fuels was derived as follows. We examined the effects of different 
C0 2 taxes on the ratio of carbon to noncarbon fuels in the runs 
presented in Nordhaus (1979), Chapter 8. The logarithmic derivative of 
the ratio of the two fuels to the ratio of their prices was somewhat 
greater than 1.5 in absolute value. We reduced the elasticity to 1.2 
to allow for the tendency of LP models to "overoptimize." The alter- 
natives were set above and below the important boundary elasticity of 
1. It must be noted that the empirical basis for this parameter is as 
weak as any we rely on. 

Turning now to the parameters in Equation (6b) , estimates for g^, 
g^, and R are required. Estimates of world fossil fuel reserves vary 
widely according to the assumptions that are made about economic feas- 
ibility. Table 2.11 registers the variety from which our estimates were 
drawn. The low range includes only proven reserves that will certainly 
become economically feasible in the foreseeable future. The middle 
range captures a large increment of reserves that most researchers think 
will become feasible in that time span; it quadruples the low range by 
including difficult oil deposits and extensive use of cleansed coal. 
The upper range adds a small percentage of potential shale availability 
to the supply and puts world resources well beyond quantities that will 
be consumed over the span of our study the next 125 years. 



123 
TABLE 2.11 World Resources of Fossil FuelsS. 



A. 


Certain Economic Feasibility (low R) 








I IAS A 


2.7 


x 10 12 mtce 




WAES 


3.2 


x 10 12 mtce 




Value employed for low R 


3 


x 10 12 mtce 


B. 


Probable Economic Feasibility (middle R) 








I IAS A 


11 


x 10 12 mtce 




IEA (high) 


12.2 


x 10- 1 - 2 mtce 




IEA (low) 


12.0 


x 10 12 mtce 




Value employed for middle R 


12 


x 10 12 mtce 


C. 


Including Shale Estimate (high R) 








Total deposits Duncan and Swanson 


144 


x 10 3 - 2 mtce 




Value employed for high R (= middle R + 








0.07 shale) 


22 


x 10 12 mtce 



^Sources: IIASA (1981) ; Energy; Global Prospects , Workshop on 

Alternative Energy Strategies (WAES) (1977) ; Reilly et al. (1982) ; 

Duncan and Swanson (1965) . 

-These numbers are consistent, component by component, with other 

incomplete data found in Moody and Geiger (1975) , and World Energy 

Conference (1978) . 

The inclusion of shale allows for incredible availability of fossil 

fuel. The 7% utilization rate, chosen rather arbitrarily, generated a 

resource constraint that was always nonbinding through the year 2100. 



The procedure for computing g^ and g 2 is more involved. For 
simplicity, first of all, g 2 is set equal to 1; manipulating g^ 
provides more than enough flexibility. A range of prices for fossil 
fuel in some future time after an arbitrary Ri mtce of fossil fuel 
had been consumed, is then constructed. Denoting those prices by PJ 
and the various reserve estimates cited above by_R k , a collection of 
g^ values, now clearly dependent on both Pj and R^, are computed 
according to 

P.. - 9 + g^jrk) [\/(\ - R 1 )lr (8) 

where j and k index high, middle, and low values for Pj and R^, 
respectively. Figure 2.10 shows that this procedure generated three 
possible paths for each of the three R^; i.e., nine separate 
specifications of the g^(j,k) and thus nine specifications of 
Equation (6b) . Table 2.12 meanwhile records the prices estimated by 
several studies for RI = 1100 x 10 9 mtce. It is a value chosen 
because of the availability of these price projections, and the table 
shows how the necessary aggregate prices are computed. Several other 
studies cited either prices without aggregation weights or consumption 
mixes without prices, so they were of little use. It is, nonetheless. 



124 



121.5- 



121.0- 



120.5- 



I 

I 120.0- 



1 

O 

-o 
ir> 

55 



119.5- 



119.0- 



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118.0-3 

LL 
O 
LU 
O 

QC 117.5-3 

CL 



117.0- 



116.5- 



116.0 



Low 

Middle 

High 




20 40 60 80 100 

CUMULATIVE FOSSIL FUEL EXTRACTION 



i 
120 



140 



FIGURE 2.10 Secondary (retail) price of fossil fuel as a function of 
cumulative fossil fuel extraction. Prices are 1975 dollars per metric 
ton of coal equivalent. Cumulative extraction is measured from 1975 in 
billion metric tons of coal equivalent. In the terms of this section, 
these are price paths for a given R k and P^ * (P lf p Mf P H ) 
for R. D L M H 



125 

TABLE 2.12 Primary (Wholesale) Fossil Fuel Prices when Cumulative 
Extraction from 1975 (R^ - 1100 x 10 9 mtce 



Source of 
Projection 5 - 


Oil* 


Gas^ 


Coal- 


Aggregatg 
Price' 


IIASA (high) 


$130(0.33) 


$70(0.29) 


$25(0.39) 


$73/mtce 


IIASA (low) 


$150(0.34) 


$65(0.23) 


$25(0.43) 


$76/mtce 


IEA (low) 


$140(0.28) 


$65(0.18) 


$25(0.54) 


$64/mtce 


IEA (high) 


$140(0.34) 


$65(0.18) 


$25(0.47) 


$71/mtce 


RFF 


$123(0.38) 


$75(0.27) 


$40(0.35) 


$81/mtce 


Mean 








$73/mtce 


Standard deviation 








$5.7/intce 


Cell extremes 








$65; $81/mtce 



^Sources: IIASA (1981), Reilly et al. (1982). 

The proportions of each source in the total consumption are given in 
the parentheses. 

Aggregate prices are computed as weighted sums of the components, 
with the weights being the proportions noted in the parentheses. Thus, 
for IIASA (high) (0.33) ($130) + (0.29) ($70) + (0.39) ($25) - $73/ratce. 
^Many studies did not provide a full range of necessary data. A 
compilation of 23 surveys of projected oil prices produced during the 
1981 Stanford International Energy Workshop did, however, provide a 
good sample of oil price expectations through the year 2000. Extrapo- 
lating those data through 2030 [the year when most studies see R(t) = 
1100 x 10 9 mtce] using the range of weights listed here produced a 
much wider range extending from $43/mtce to $106/mtce. These are rough 
estimates, of course, but lead to some widening later. 



interesting to note that interpolating between the estimated oil prices 
used in this study for 2025 (and contained in Table 2.12) and the 1981 
oil prices provides us a benchmark for comparison. This benchmark fell 
in the third decile of 23 studies (i.e., lower end) compiled during the 
Stanford International Energy Workshop of 1981 (Energy Modeling Forum, 
Stanford University) . 

Even with these data collected, our work on the price equation for 
fossil fuel is not completed. Section 2.1.2.1 outlines an approximation 
procedure that allowed both the simplicity of the nested production 
function recorded in Equation (1) and the flexibility of being able to 
vary the elasticity of substitution between energy and labor across 
time. It was an adjustment made necessary by a desire to incorporate a 
source of dynamic uncertainty into the model that could well loom large 
in the balance of this century. Much in the same spirit, we now need 
two similar adjustments in the fossil fuel equation. The first is 
designed to preserve the structure of Equation (3) even as the short- 
term effects of restricted oil supplies were recognized. The second 
gives society enough foresight to prepare for the imminent exhaustion 
of fossil fuel supplies. 



126 

The need for the first adjustment can be seen by looking at the 
very recent past. The model, as presented above, allows instantaneous 
substitution into and out of aggregate energy and between its carbon 
and noncarbon components each year in response to changes in relative 
input prices. While this is a conventional assumption for long-range 
growth models in which people are presumed to predict price movements 
accurately and plan accordingly, it does not conform well to the uncer- 
tain world that has confronted energy consumers since 1975. The 
dramatic disruption in world oil supplies caused by the advent of OPEC 
in 1973, the events in Iran, the oil glut, and the decontrol of gas and 
oil prices in the United States are all examples of factors that have 
contributed to the uncertainty; and their net effect has been to 
increase the primary price of carbon-based fuel from $35/mtce in 1975 
to $76/mtce in 1981. Investment decisions taken in 1975 were, however, 
made in response to 1975 prices and 1975 expectations. Much of the 
world's present capital stock was, in fact, put into place before the 
oil shocks of 1973. The decisions that produced these investments were 
clearly not made with the type of accurate foresight required in the 
model. Nor can it be presumed that instantaneous substitution would 
have brought all the existing capital up to date relative to current 
energy prices. Thus, there exists a need to provide a longer reaction 
time at the beginning of the model to reflect the difficulty faced by 
most consumers in responding to such enormous price changes. 

One possible adjustment would involve making alterations in the 
production function, but that course is again rejected to avoid com- 
plexity. Rather than produce the complications of more complex 
intertemporal substitution, we modify the early fossil fuel prices 
against which consumption decisions would be made. For the first 25 
years of each run, in particular, a linear combination of projected 
current fossil fuel prices (computed from 1981 prices) and the lower 
1975 prices is employed to slow the rate of growth of fossil fuel 
prices; the result is a reduction in the reaction to higher fossil fuel 
prices mandated by Equation (3). After the year 2000, however, this 
delayed reaction is stopped and decisions are assumed to be made on the 
basis of prevailing fuel prices. 

More specifically, the 1975 primary price of fossil fuel ($35/mtce) 
is used in conjunction with the price ranges computed for R! - 1100 x 
10* mtce to compute the appropriate gj_ coefficients. They are 
recorded here in Table 2.13. This computation, with the R^ price 
range expanded to $43, $73, and $103 per mtce to reflect the larger 
dispersion of the Stanford estimates of oil prices, is appropriate 
because the price estimates on which the range was based were made 
under 1975 expections. Nonetheless, the primary price of fossil fuel 
did reach $76/mtce by 1981, and distribution costs did rise by $40/mtce 
from 1975 through 1981. These figures, therefore, are used as initial 
conditions for the long-term supply equation; i.e., the equation 

,9o\ 

(t)t] + 40 (9) 




fully specifies the long-term price equation for fossil fuel. Still, 
the point of this adjustment is that imposing these inflated prices in 



127 
TABLE 2.13 The g Parameters of P c (t) 



Price at 1- g Probability 



3200 x 10 9 mtce 


$43/mtce 


13.8 


0.06 


3200 x 10 9 mtce 


$73/mtce 


65 


0.13 


3200 x 10 9 mtce 


$103/mtce 


118 


0.06 


11000 x 10 9 mtce 


$43/mtce 


72 


0.13 


11000 x 10 9 mtce 


$73/mtce 


342 


0.24 


11000 x 10 9 mtce 


$103/mtce 


612 


0.13 


21000 x 10 9 mtce 


$43/mtce 


145 


0.06 


21000 x 10 9 mtce 


$73/mtce 


687 


0.13 


21000 x 10 9 mtce 


$103/mtce 


1230 


0.06 



^Source: Tables 2.11 and 2.14, Equation (6b) , and the text of this 

section. 

kin 1975 U.S. dollars. 



1981 would not have been consistent with the spontaneous flexibility of 
the production function. Since the relevant secondary price of fossil 
fuel in 1975 is $62/mtce and not $116/mtce f the operative fossil fuel 
price for the first 25 years is adjusted linearly according to 

[P C ( t )] = [(25 - t)/25]62 + [t/25]P C (t). 

Notice, as illustrated in Figure 2.11, that [P C (t)]' and P (t) are 
therefore coincident only after the year 2000. 

The second adjustment is necessary to preclude the possibility that 
the world would unexpectedly exhaust all of its fossil fuel reserves. 
The notion here is that there exists a "backstop" technology (such as 
solar or fusion) that should become economically feasible before 
exhaustion and that entrepreneurs would provide that technology before 
the economic effects, perhaps collapse, that unexpected exhaustion 
would create. For our purposes, we model the backstop as a gradual 
contraction of reliance on fossil fuel once its price climbed to levels 
in excess of four times the price of nonfossil fuel. The multiple is 
selected to match current estimates of the cost of generating hydrogen 
from conventional sources; the subsequent rate of decline of fossil 
fuel consumption is assumed to be roughly 6% per year and is estimated 
from preliminary runs in which the supply of fossil fuel was exhausted 
in the absence of the backstop. 

Consideration of the airborne fraction is the final order of busi- 
ness. Estimates from a variety of experts are cited in Clark (1982) , 
but we found that they were mostly the products of statistically 
inefficient estimation procedures and highly sensitive to assumptions 
made about the contribution of carbon dioxide to the atmosphere from 
the biosphere. The latter sensitivity reflected misspecification of 



128 



120^ 



115- 



110- 



105- 



100- 



o- 

0) 



- 90- 

_ 

1 

K 85- 

05 



^ 80< 

LL 



75- 



LL 

S 71 

y 

DC 

Q. _ 
65- 



55 



P c (t) 



'[P c (t)] f 



50 



1975 



2000 



2025 



i ' 

2050 



2075 



2100 



YEAR 

FIGURE 2.11 Price of fossil fuel (1975 dollars per ton of coal 
equivalent). A comparison of P c (t) and [P c (t) ] '--the adjustment 
required to accommodate the rapid increase in fossil fuel prices from 
1975 through 1981. 



129 

the appropriate estimating equation and led us to our own study. At 
this point f we advance a maximum likelihood estimate of the marginal 
airborne fraction equal to 0.47, given an average seepage of 0.1% of 
ambient carbon dioxide into the oceans and an annual contribution from 
the biosphere of 1 Gt of C (the mean estimate). We differentiate 
between a marginal airborne fraction (the fraction of current emissions 
that remain in the atmosphere during their first year) and an average 
airborne fraction because of the seepage factor. The contribution of 
each year's emissions decays over time in our model, and that decay 
will have implications later when we turn to consider emission taxes as 
policies with which to address the carbon dioxide problem. The extreme 
cell estimates of the marginal fraction are also computed to discretize 
the usual normal distribution around a regression coefficient; 0.38 
emerges as the low estimate, and 0.59 is the upper extreme. 

2.1.2.2.1 Adjustment of Subjective Uncertainty 

The inability of individuals, even those with statistical training, 
to deal efficiently with uncertainty about the future has come under 
increasing scrutiny. Studies in both the economic and psychological 
literature have argued, in particular, that individuals tend to 
underestimate the uncertainty about events.* For present purposes, two 
reasons for these systematic errors are relevant. First, when people 
look to previous studies in seeking guidance for their own views, they 
may place too much weight on the early studies. If one were then to 
view the range of resulting estimates as an indication of the true 
uncertainty, the computed variance would be too small. 

A simple illustration of this phenomenon, sometimes known as the 
wine-tasting problem, can make this point. Suppose that a sample of 
two independent observations, x^ and X2, were taken from the same 
distribution, and let that distribution be normal with mean < and 
variance a 2 . Each x^ would therefore be independently, normally 
distributed with mean ic and variance a 2 . The sample mean, 5c = (x^ + 
x 2 )/2 would then be an unbiased estimate of K, and S 2 (X! - x) 2 + 
(X2 - x) 2 would be an unbiased estimate of a 2 (i.e., |E[S 2 ] f = 
a 2 ) . If, however, the second scientist looks over the shoulder of the 
first, he might allow his judgment to be influenced. Say that the reports 
of observation x 2 were weighted by x^ so that reported values (y) are 
y l = x l and ^2 = ax 2 + d "" a)xjL. The reported variance would 
decline. The reason is that the y 2 would display a variance 

o 2 (y 2 ) = a 2 a 2 + (1 - a) 2 2 = [1 + 2 (a 2 - a)] a 2 < a 2 , for < a < 1. 



*See Arrow (1982) for a summarizing review of both. 



130 

So f the variance of the reported values is biased downward from a 2 . 
The infusion of judgment allows the first researcher's result to 
influence the second , and the observed variance is smaller than the 
underlying variance 

Second, people seem reluctant to accept the true uncertainty inherent 
in small samples. Several studies report that individuals frequently 
base their expectations on one observation even when they are aware 
that historical experience has been widely varied.* The estimates for 
the more distant periods recorded in this section seem to suggest such 
a telescoping of uncertainty. In some cases/ the range of estimates 
declined as the forecast period increased, even though the passage of 
time should have increased the uncertainties. It appears that, in the 
face of higher uncertainty, scientists may look to each other for 
guidance. 

To correct for the resulting tendencies to underestimate the degree 
of uncertainty, estimate ranges are expanded around the computed means; 
i.e., the ranges are adjusted either to keep the ranges from contract- 
ing over time or to make them consistent with historical experience. 
The adjustments are based on our judgment but are undertaken only if 
they could be justified by one of these two rationales. 

Table 2.14 presents the results of this procedure. The population 
growth ranges after the year 2025 are, for example, expanded to main- 
tain the 0.5% deviation computed for the 2000-2025 period. The later 
productivity ranges are similarly expanded to match the uncertainty 
found in the first two periods. Energy price ranges are, finally, 
widened in response to the enormous political and economic uncertainties 
inherent in the world energy market. The survey of 23 projections for 
oil prices collected during the 1981 Stanford International Energy 
Workshop (ranging from 10% reductions to 100% increases in the price of 
imported crude oil) provide some very rough guidance for our energy 
price uncertainty (Manne, 1982) . 



2.1.2.3 Results 

2.1.2.3.1 Levels and Uncertainties of Major Variables 

Four types of experiments are conducted with the fully specified 
model. In the first, we investigate not only the most likely paths of 
emissions and concentrations but also the inherent uncertainty that 
surround those projections* This is accomplished by taking 1000 random 
samples from the 3* different trajectories. The results of a sample 
of 1000 runs are recorded in Table 2.15. Figures 2.12 through 2.16 
plot the first 100 of those runs for some of the more important vari- 
ables. And Table 2.16 records the annual growth rates of the most 
likely path for those variables. Notice that these rates of growth, 
particularly those for energy consumption and GNP, conform well with 



*See Arrow (1982) and the sources cited therein. 



131 
TABLE 2.14 Adjusted Ranges!. 



1975-2000 2000-2025 


2025 
and Beyond 



A. Population Growth 

High 2.0% 1.6% 0.8% 

Middle 1.7% 1.1% 0.3% 

Low 1.4% 0.6% -0.2% 

B. Productivity Growth 

High 3.4% 0.9% 0.1%(0.5%) 

Middle 2.3% 1.6% 1.0% 

Low 1.2% 2.3% 1.9%(1.5%) 

C. Non fossil Fuel Price Growth 

High 2.0% (1.3%) 1.0% (0.5%) 1.0%(n.a.) 

Middle 0.5% 0.0% 0.0% 

Low -1.5%(-0.2%) -1.0%(-0.5%) -1.0%(n.a.) 

D. Aggregate Carbon Emissions no change 

E. Fossil Fuel Prices for R^ - 1100 x 10 9 mtce 

High $103/mtce ($81/mtce) 

Middle $ 73/mtce 

Low $ 43/mtce ($65/mtce) 

F. Airborne Fraction no change^ 



^Source: Previous tables and the present text. Unadjusted figures 
are indicated in parentheses when adjustments to widen the ranges have 
been made. 

J2The uncertainties cited were measurement problems and were 
biometrically evaluated from the' carbon dioxide literature; they were 
not subject to the types of underestimation cited here. 



the averages of the projections cited in Ausubel and Nordhaus (Section 
2.2) through the year 2025. The 50-year averages predicted by the 1000 
runs are, in fact, 2.1% and 3.3% for energy and GNP; the averages for 
the previous studies are 2.4% and 3.4%, respectively. The results 
presented here should not, therefore, be considered to be the products 
of a model that embodies radically different expectations about 
economic growth than the consensus of professional opinion. 

The uncertainty surrounding the average path is, however, quite 
striking. The measured standard deviations of all variables expand 
over time, and that expansion is sometimes dramatic. For carbon emis- 
sions and concentrations, in particular, a fair amount of certainty 
through the year 2000 balloons to the point where, by 2100 , standard 
deviations of their projections equal 60% and 23% of their means, 
respectively. Put another way, the extreme values for concentrations 
run from 377 ppm to 581 ppm in the year 2025, and from 465 ppm to 2212 
ppm in the year 2100] Those interested in the actual distributions of 



132 









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I 



(o 

O 
o 

i 



1.0- 




1975 



2000 



2025 



2050 



2075 



2100 



YEAR 



FIGURE 2.12 Fossil fuel consumption for 100 randomly drawn runs 
(billion metric tons of coal equivalent per year) . 



134 



2500.0H 



g 1250.0 



O 

8 

O 

DC 
LJJ 

85 

O 



625.0- 



312.5- 




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780 



370 



1975 



2000 



2025 



2050 



2075 



2100 



YEAR 



FIGURE 2.13 Atmospheric concentration (parts per million) of carbon 
dioxide for 100 randomly drawn emission runs. The numbers on the 
right-hand side indicate the mean concentration for the year 2100 and 
the extreme high and low outcomes. 



emissions and concentrations for critical years are referred to Figures 
2.17 and 2.18. 

The model presented here finds that carbon dioxide emissions are 
likely to grow steadily over the next century or so, with an atmo- 
spheric concentration reaching 600 ppm, in our most likely case f 
shortly after 2065. If we call attainment of 600 ppm a "doubling, B our 



135 



896H 



394 



113 




1975 



2000 



2025 



2050 



2075 



YEAR 



FIGURE 2.14 Gross world product for 100 randomly drawn runs (trillion 
1975 dollars) . 



estimates indicate a doubling time longer than some earlier studies. 
This slower buildup arises primarily because we estimate a greater 
sensitivity of fossil fuel consumption to rising fossil fuel prices. 
But while this average result suggests a considerable time before a 
CO 2 doubling, our analysis also shows a substantial probability that 
doubling will occur much more quickly. Looking at the distribution for 
the year 2050, in fact, our results show a 27% chance that doubling 
will already have occurred. Unless this uncertainty can be reduced by 



136 




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137 



500.00- 



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o 



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o 
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0.80- 



0.16- 




209 



31.1 



2.3 



1975 



2000 



2025 2050 

YEAR 



2075 



2100 



FIGURE 2.16 Nonfossil fuel consumption for 100 randomly drawn runs 
(billion metric tons of coal equivalent per year) 



138 

TABLE 2.16 Annual Growth Rates of Critical Variables (percent per 
annum) 



Variable 


1975- 
2000 


2000- 
2025 


2025- 
2050 


2050- 
2075 


2075- 
2100 


GNP 


3.7 


2.9 


1.5 


1.5 


1.5 


Energy consumption 


1.4 


2.7 


1.2 


1.1 


1.2 


Fossil fuel consumption 


0.6 


2.5 


0.9 


0.5 


0.4 


Nonfossil fuel consumption 


5.6 


3.1 


1.8 


2.0 


2.0 


Price of fossil fuel 


2.8 


0.3 


1.2 


2.9 


1.1 


Price of nonfossil fuel 


0.5 


0.1 


0.1 


0.1 


0.1 


C(>2 emissions 


0.6 


2.6 


1.2 


0.9 


0.4 


Concentrations 


0.3 


0.6 


0.8 


0.8 


0.8 



3rhese are calculated as the probability weighted means of the 
100 random runs* 



further research, it would appear to be unwise to dismiss the possibil- 
ity that a CC>2 doubling may occur in the first half of the twenty- 
first century. Perhaps the best way to see this point is to refer back 
to Figure 2.4. There, only five paths are drawn, but it is clear that 
doubling occurs before the year 2050 along two of them, the 95th and 
75th percentile paths* 

Finally, it is surprising that the backstop technology comes into 
play on 11% of the runs, with the earliest transition occurring in year 
2054. Recall that a backstop technology is invoked when fossil fuels 
are almost completely exhausted. Exhaustion (and appearance of the 
backstop) usually requires a combination of variables that include low 
fossil fuel reserves, high productivity growth, high population growth, 
and small substitution possibilities out of carbon-based fuel. The 
share of GNP devoted to energy always rises sharply during the period 
immediately preceding transition to the backstop and thus conforms well 
to the notion that the conversion to a backstop technology will be 
expensive. Transition to the backstop then causes carbon emissions to 
fall to roughly 5% of their peak over the next 25 years. 

2.1.2.3.2 Sources of Uncertainty 

A second experiment is designed to determine which of the 10 sources 
of uncertainty was most important in producing the ranges that have 
just been noted; it was conducted in two ways. In the first, each of 
the 10 random variables are, in turn, set equal to their two extreme 
values while all the others are fixed at their middle setting. In that 
way, the individual contributions of each source to the overall uncer- 
tainty of the projections is measured and compared. The second column 
of Table 2.1 records an index of the individual standard deviations 
that emerged. The second method is, in effect, the converse of the 



139 

first. Each random variable is, in turn, held at its most likely (or 
"middle") value while random samples are taken across the other nine; 
the resulting reduction in the uncertainty is then taken to be a reflec- 
tion of the marginal or incremental contribution of the fixed variable 
to overall uncertainty. The first column of Table 2,1 records an index 
of this measure. 

Notice that both measures produce similar rankings. The ease of 
substitution between fossil fuels and nonfossil fuels rank first in 
both; this is an area that has thus far been almost ignored in prior 
study of the carbon dioxide problem. Second in both lists is the rate 
of productivity growth, but the importance of this variable is intui- 
tively clear and has been apparent for some time. Below these two, a 
second echelon grouping of four variables appears in both columns: ease 
of substitution between labor and energy, extraction costs of fossil 
fuels, technological change in the energy sector, and the airborne 
fraction. The last member of this list has been heavily studied over 
the past few years, but even our wide range of uncertainty only pushed 
it into the middle of the ranking according to either scale. The fuel 
mix and the rate of growth in the population form a third grouping. 

The bottom factors in terms of contribution to uncertainty are trends 
in relative energy costs and world fossil fuel resources. Holding 
these last two fixed actually produced higher variation in concentra- 
tions in 2100. The cost variable effect is small and probably cannot 
be statistically distinguished from zero, but the resources effect is 
pronounced. It is, however, easily explained. The backstop technology 
is invoked only when world resources are set at their lowest value. 
And when the backstop is imposed, emissions fall quickly to zero and 
concentrations tend toward roughly the same number. Removing the back- 
stop as a possibility by removing the possibility that world fossil 
fuel resources might be quite, small therefore removes circumstances 
that have a serious dampening effect on the range of possible atmo- 
spheric concentration. An expanded variance should therefore be 
expected. 

2.1.2.3.3 Validation 

Our third area of experimentation poses the problem of validating 
our results. Validation is, of course, a major issue arising in the 
estimation and use of very-long-run economic and energy models where 
the time period over which the data are available is typically much too 
short to permit testing and validating by usual statistical techniques. 
Moreover, economic systems evolve and mutate over time, so even models 
that use classical statistical time series tests would be suspect. Our 
time frame 125 years clearly heightens concerns about these concerns. 
This raises the question of whether we have accurately estimated the 
uncertainty of future events by looking at each of the variables indi- 
vidually, imposing distributions on them and expanding those distribu- 
tions to account for the likelihood that scientists systematically 
overestimate the confidence in their results. Two types of tests are 
run to attempt to answer this question. 



140 




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144 

TABLE 2.17 Alternative Estimates of the Uncertainty of Carbon Dioxide 
Emissions (Calculated as the Standard Deviation of the Logarithm of 
Emissions) 



Statistical (Calculated 
Time from from Historical Data) 
Last Data (t) Static! Dynamic^ Model 






0.005 


0.06 


0.04 


25 


0.14 


0.50 


0.56 


50 


0.28 


0.75 


0.61 


75 


0.42 


0.76 


0.67 


100 


0.56 


0.99 


0.68 


125 


0.70 


n.a. 


0.85 



^Calculated as t x se(g), where t is time in future and se(g) is the 
standard error of g in a regression log (emissions) - a + gt, over the 
sample period 1960-1980. The equation was estimated assuming 
first-order autocorrelation of residuals. 
=Calculated as the standard deviation of a forecast of log 
(emissions) t periods in the future or past. The number of 
nonover lapping samples were 10 for t = 0, 9 for t = 25, 6 for t 50, 4 
for t = 75, and 2 for t = 100. 

^Calculated as the standard deviation of log (emissions) for 100 
randomly selected runs. 



We first use classical prediction theory to estimate the prediction 
errors that are consistent with the data over the period 1960-1980 
(see, for example, Johnston, 1972; or Malinvaud, 1980). Under this 
approach, we assume that there was a "true" growth rate of emissions, 
g, and that the data over the 1960-1980 period are an unbiased sample 
of that true growth rate. The mean growth rate over this period is 
3.67% per annum, and the standard deviation of the growth rate is 
0.561% per annum. Using this approach, we show in column 2 of Table 
2.17 the estimated standard errors of emissions that would be expected 
over forecast periods extending further and further into the future. 

In the second approach, the historical data are used to provide 
out-of-sample forecasts. (This technique has been used infrequently. 
For an example as well as further discussion, see Fair, 1978.) Under 
this approach, we estimate a growth trend for each of five 21-year 
periods (1860-1880, 1880-1900, 1900-1920, 1920-1940, 1940-1960). On 
the basis of the estimated trend functions, we then forecast into the 
future (unknown then but known now) through 1980. Thus we obtain, 
respectively, 100, 80, 60, 40, and 20 years of out-of-sample forecasts. 
The same procedure is then used to "backcast, 11 that is, to fit functions 
to recent data and then to project backward into time what emissions 
should have been. In the backcast exercises, for example, we fit a 



145 

trend function to the data for 1960-1980, then use that estimated 
relation to calculate emissions over the period 1860-1960. Again, five 
21-year trends are estimated and five different sets of backcasts are 
constructed. 

From the 10 sets of forecasts and backcasts , we construct a set of 
out-of-sample errors, 0, 25, 50, 75, and 100 years away from the 
sample future or past. The root-mean-squared errors are then cal- 
culated; they are labeled as "dynamic" statistical error forecasts and 
are shown in column 3 of Table 2.17. 

The backcast procedure may at first appear bizarre. It is an 
implication of the model we are using, however, that estimates of the 
structure are equally valid forward and backward into time. This 
implication arises because the trend model has no explanatory variable 
but time, as well as because the trend model assumes that there is no 
change in the underlying economic structure. It should be emphasized, 
however, that use of this type of trend extrapolation model to forecast 
emissions in no way is an endorsement of such a technique for all 
purposes. We are employing it here only for validation purposes. 

The results of this validation test indicate that the error bounds 
for emissions estimated by the model, recorded in column 4 of Table 
2.17, are within the bounds generated by the two historical error 
estimation procedures. In general, the model produces error bounds 
greater than the classical statistical technique shown in column 2, but 
smaller than the dynamic estimates shown in column 3. 

If we were to choose between procedures for estimating errors, we 
would be inclined toward the dynamic rather than the static as a 
realistic estimate of forecasting uncertainty. The reason for this 
inclination is that the static estimate assumes that there is no change 
in the underlying structure of the economy, so that future growth rates 
are drawn from the probability distribution generated by the in-sample 
growth rates. The dynamic model, on the other hand, recognizes that 
there is evplution in the structure of the economy, so that the distri- 
bution from which we draw observations is likely to drift around over 
time. Assuming that the pace of economic structural change over the 
next 100 years will be about as rapid as that over the last 100 years, 
the dynamic estimates give a better estimate of the realistic error 
bounds for a forecast of the future. To the extent that careful struc- 
tural modeling allows us to improve on a naive extrapolation of trends 
which is after all the major point of economic and energy models of the 
kind we introduce herethe error bounds of the model should be an 
improvement over the dynamic error bounds. And, finally, it should be 
noted that these exercises were conducted after the model had been 
constructed and estimated; no tuning of the model has been undertaken 
to bring it in line with either series recorded in Table 2.17. 

2.1.2.3.4 Policy Experiments 

Our final set of experiments considers the possibility that govern- 
ments will intervene to curtail C0 2 emissions. There are clearly a 
wide variety of approaches to discouraging fossil fuel combustion and 
C02 emissions. Some might take the form of taxes on production or 



146 

consumption of carbon-based fuels; nonfossil sources might be encour- 
aged; countries that have large coal reserves might place export 
limitations or heavy taxes on coal exports. Government might agree to 
national CC>2 emissions quotas and then enforce these in a wide 
variety of ways. At this point, we do not pass on the likelihood or 
desirability of these different policies rather, we attempt to 
investigate their impacts. 

For purposes of analysis, it is convenient to convert all policies 
that discourage use of C(>2 into the carbon-equivalent taxes. As an 
example, say that a 2x% tax on carbon fuels would always produce an x% 
reduction in their use. We would then use this hypothetical formula as 
a way of representing any quantitative restriction on carbon-based 
fuels. Whatever the set of policies, we can derive the tax rate on 
fossil fuels that would produce the same restraint on C0 2 emissions. 
This equivalent tax is the carbon-equivalent tax investigated here. We 
consider only the most likely run the case in which all the random 
variables are set equal to their middle values. Because of the crude- 
ness of the policy, the results quoted below should be viewed as being 
extrem'ely tentative. 

Five different taxes are imposed on the supply equation for fossil 
fuel. Two are taxes increasing from zero to $20 per mtce over the 
course of 10 years and then declining back to zero over the next 
decade; one pulse begins in the year 1980, and the other begins in the 
year 2025. For a second set of runs, permanent taxes of $10 per ton 
are introduced linearly over the first 20 years. One begins in the 
year 1980, and the other begins in the year 2025. A final tax, modeled 
after the 100% control case presented in Nordhaus (1979) and called the 
"stringent tax," imposes a permanent tax that rose linearly beginning 
in the year 2000 from zero to $6 per mtce by the year 2020, then to $68 
per mtce by the year 2040, and finally to $90 per mtce by the year 
2060. These are all illustrated in Figure 2.19. 

Table 2.18 and Figures 2.20 and 2.21 show the results of the tax 
runs. Notice there that while the pulse taxes accomplish very little, 
the permanent tax initiated in 2025 is the most effective among the 
first four alternatives. This paradox is explained as follows: burning 
more fossil fuel early and postponing C0 2 reductions lowers the even- 
tual C0 2 concentration because it allows the atmosphere to cleanse 
itself slowly. To see this, recall that our model allows for a slow 
seepage (1 part per 1000 per year) of ambient carbon dioxide into the 
deep oceans. Earlier emissions, therefore, gradually disperse into the 
deep oceans and produce end-of-period concentrations that are somewhat 
lower . 

The major conclusion concerns the extent to which concentrations 
appear to respond to taxes. As can be seen, a $10 per ton tax accom- 
plishes only a modest reduction in CO 2 concentrations. Should 
emissions restraint become desirable, it will take extremely forceful 
policies to make a big dent in the problem. Even the "stringent 



" 



147 



90 -J 



80 



70 



60 



o 

f 

J2 50 - 

o 
o 

LD 
CD 

~ 40- 
X 



30 



20 



10- 




TIME 
Pulse 1980 
Pulse 2025 
Stringent 
Permanent 1980 
Permanent 2025 



A 





1 \ 
1 \ 




1 \ 




1 \ 




1 \ 




t \ 


- 


1 Y 




1 S * 




1 ' \ 






I 


Is 




1975 



2000 



2025 



2050 



2075 



2100 



YEAR 



FIGURE 2.19 Taxation on carbon fuel price (1975 dollars per ton coal 
equivalent). The time tracks of a stringent tax and four alternative 
$10 per ton of coal equivalent taxes? the temporary taxes peak at $20 
to accommodate the model 



148 

TABLE 2.18 Concentrations and Emissions along the Likelihood Path 
under Various Taxes on Carbon-Based Fuels 

1975 2000 2025 2050 2075 2100 

Concentrations 

Base 341 368 428 516 633 780 

1980 Permanent^ 341 367 423 506 617 759 

1980 Peaked^ 341 367 423 513 633 778 

2025 Permanent^ 341 368 428 513 632 763 

2025 Peaked^ 341 368 428 521 638 783 

Stringent Taxes 341 368 425 487 561 661 



Emissions 














Base 


4.61 


5.54 


10.3 


13.3 


17.5 


20.0 


1980 Permanent 3 . 


4.61 


5.06 


9.5 


12.5 


16.7 


19.5 


1980 Peaked^ 


4.61 


5.31 


10.3 


14.0 


17.6 


19.4 


2025 PermanentS. 


4.61 


5.54 


10.3 


12.3 


16.5 


19.3 


2025 Peaked^ 


4.61 


5.54 


10.3 


13.1 


17.3 


19.9 


Stringent Taxes 6 - 


4.61 


5.54 


8.4 


7.9 


10.7 


13.9 



3A permanent tax of $10 per ton imposed linearly beginning in 1980 

and reaching its full value by the year 2000. 

HA pulse tax of 20 years 1 duration beginning in 1980, climbing 

linearly to $20 per ton by 1990 and then falling to zero by the year 

2000; it averages $10 per ton. 

HA permanent tax of $10 per ton imposed linearly beginning in 2025 

and reaching its full value by the year 2045. 

HA pulse tax of 20 years 1 duration beginning in 2025, climbing 

linearly to $20 per ton by 2035 and then falling to zero by the year 

2045; it averages $10 per ton. 

A gradually increasing tax rising linearly from zero to $8 per ton 

between 2000 and 2020, from $8 to $68 per ton between 2020 and 2040, 

from $68 to $90 per ton between 2040 and 2060, and remaining at $90 per 

ton thereafter. This tax was drawn from Nordhaus (1979, Chapter 8). 



taxes, which would place 60% surcharges on the prices of fossil fuels, 
did not prevent doubling before 2100 in our most likely case.* 



*Nordhaus (1979) presented an earlier estimate of the taxes needed 
to curtail the carbon dioxide buildup in an optimizing linear program- 
ming framework. In that calculation the potency of carbon taxes was 
very close to the estimates here. The estimate here is 0.46% reduction 
in 2100 carbon dioxide concentration per $1 of carbon tax, while in the 
earlier work the estimate was 0.36% reduction per $1. Note that 
comparison is not completely appropriate because these reactions are 
likely to be nonlinear. 



149 



0- 



-1- 



-2- 



-3- 



c 

a 




UJ 

z 

o 

CD 
DC 



-7- 



-8- 




Permanent Beginning 
in 198O 



Permanent Beginning 
in 2020 



Temporary Beginning 
in 1980 



Temporary Beginning 
in 2020 



Stringent 
Taxes 



Pulse 1980 

Permanent 1980 

Stringent 

Pulse 2025 

___ Permanent 2025 



1975 



2000 



2025 



2050 



2075 



2100 



YEAR 



FIGURE 2.20 Plot of carbon emission versus time for taxecl runs, 
Deviation in emissions from the base run for various taxes. 



It should be emphasized that this conclusion about the potency of 
C02 taxes (or their regulatory equivalents) is extremely tentative. 
It is based on a model for which many of the parameters are known 
imperfectly. On the other hand f the model's conclusions appear to 
confirm results of a completely independent model/ as reported in the 
last footnote and those of Edmonds and Reilly (1983) . 



150 



Temporary 
Beginning 
in 2020 



Permanent 
Beginning 
in 2020 



Temporary 
Beginning 
in 1980 



Permanent 
Beginning 
in 1980 



Stringent Taxes 



Pulse 1980 
Permanent 1980 
Stringent 
Pulse 2025 
Permanent 2025 




-90 



-100 



-110 



-120 



1975 



2000 



2025 



2050 



2075 



2100 



YEAR 



FIGURE 2.21 Effect of carbon taxes on atmospheric concentration (parts 
per million per year) . Deviation of run from base run without carbon 
taxes. 



Nevertheless, the conclusions about the potency of policy are 
sobering. They suggest that a significant reduction in the concen- 
tration of C0 2 will require very stringent policies, such as hefty 
taxes on fossil fuels. Global taxes of around $60 per ton of coal 
equivalent (approximately $10 per barrel of oil equivalent) reduce the 
concentrations of C0 2 at the end of our period by only 15% from the 
base run. Moreover, these taxes must be global; it is presumed that a 
tax imposed by only a fraction of the countries would have an effect 
roughly proportional to those countries 1 share of carbon emissions. 



151 

To the extent that such an approach can offer guidance, therefore, 
it suggests that there are unlikely to be easy ways to prevent the 
buildup of atmospheric CO2- The strategies suggested later by 
Schelling (Chapter 9) climate modification or simply adaptation to a 
high C02 and high temperature world are likely to be more economical 
ways of adjusting to the potential for a large buildup of C0 2 and 
other greenhouse gases. Whether the imponderable side effects on 
society on coastlines and agriculture, on life in high latitudes, on 
human health, and simply the unforeseen will in the end prove more 
costly than a stringent abatement of greenhouse gases, we do not now 
know* 



References 

Arrow, K. J. (1982) . Risk perceptions in psychology and economics. 

Economic Inquiry 20; 1-9 . 
Bolin, B. ed. (1981). Carbon Cycle Modelling. SCOPE 16. Wiley, New 

York. 
Brainard, W. C. (1967) . Uncertainty and the effectiveness of policy. 

Am. Econ. Rev. 57(2) . 
Clark, W., ed. (1982). The Carbon Dioxide Review; 1982. Oxford U. 

Press, New York. 
Council on Environmental Quality (1981) . Global Energy Futures and the 

Carbon Dioxide Problem. Washington, D.C. 
Duncan, D. C., and V. E. Swanson (1965). Organic Rich Shale of the 

United States and World Land Areas. U.S. Geological Survey Circ. 

523. 
Edmonds, J., and J. Reilly (1983). A long-term global energy economic 

model of carbon dioxide release from fossil use. Energy Econ. 

.5:74-88. 
Fair, R. C. (1980). Estimating the expected predictive accuracy of 

econometric models. Int. Econ. Rev. 21(2) . 
Hogan, W. W. (1980) . Dimensions of energy demand. In H. H. Landsberg, 

ed. , Selected Studies on Energy; Background Papers for Energy; The 

Next Twenty Years. Ballinger, Cambridge, Mass., p. 14. 
International Institute for Applied Systems Analysis (IIASA) (1981) . 

Energy in a Finite World: Paths to a Sustainable Future. Report of 

the Energy Systems Group of IIASA, W. HSfele, Program Leader. 

Ballinger, Cambridge, Mass. 

Johnston, J. (1972) . Econometric Methods. McGraw-Hill, New York. 
Kahn, H., W. Brown, and L. Martel (1976). The Next 200 Years. Hudson 

Institute Report, Morrow, New York. 
Keeling, C. D. (1973). The carbon dioxide cycle. In Chemistr of the 

Lower Atmosphere, N. Rasool, ed. Plenum, New York. 
Keyfitz, N. (1982). Population projections, 1975-2075. In Clark 

(1982), pp. 460-463. 
Machta, L. (1972) . The role of the oceans and biosphere in the carbon 

dioxide cycle. In O. Oyrssen and D. Jagnes, eds., The Changing 

Chemistry of the Oceans. Wiley-Inter science, New York, pp. 121-145. 
Machta, L. , and G. Telegades (1974). Climate forecasting. In Weather 

and Climate Modification, W. N. Hess, ed. Wiley, New York. 



152 

Malinvaud, E. (1980). Statistical Methods of Econometrics. 

North-Holland, Amsterdam. 
Manne, A. S. (1974). Waiting for the Breeder. Research Report 

RR-74-5. International Institute for Applied Systems Analysis 

(IIASA), Lspcenburg, Austria. 
Manne, A. S., ed. (1982). Summary Report of the International Energy 

Workshop, 1981. Stanford University Institute for Energy Studies, 

Stanford, Calif. 
Marland, G. (1982) . The impact of synthetic fuels on global carbon 

dioxide emissions. In Clark (1982), pp. 406-410. 
Meadows, D. H., D. L. Meadows, J. Randers, and W. W. Behrens (1972). 

The Limits to Growth; A Report for the Club of Rome's Project on 

the Predicament of Mankind. Universe Books, New York. 
Modeling Resource Group (MRG) (1978) . Energy Modeling for an Uncertain 

Future. Supporting Paper 2, Committee on Nuclear and Alternative 

Energy Systems (CONAES) , chaired by T. C. Koopmans. National 

Research Council, Washington, D.C. 
Moody, J. D., and R. E. Geiger (1975). Petroleum resources: how much 

oil and where? Tec'hnol. Rev. 77 1 38-45. 
National Research Council (1979). Energy in Transition 1985-2010. 

Final Report of the Committee on Nuclear and Alternative Energy 

Systems (CONAES) . National Academy of Sciences, Washington D.C. 
Nelson, R. R. , and S. G. Winter (1964). A case study in the economics 

of information and coordination: *the weather forecasting system," Q. 

J. Econ. 78(3) . 
Nerlove, M. (1965). Estimation and Identification of Cobb-Douglas 

Production Functions. North-Holland, Amsterdam. 
Nordhaus, W. D. (1977) . The demand for energy: an international 

perspective. In W. D. Nordhaus, ed., International Studies of the 

Demand for Energy. North-Holland, Amsterdam, p. 273. 
Nordhaus, W. D. (1979) . Efficient Use of Energy Resources. Yale U. 

Press, New Haven, Conn. 
Nordhaus, W. D. (1980a) . Oil and economic performance in industrial 

countries. Brookings Papers on Economic Activity 2:341-388. 

Brook ings, Washington, D.C. 
Nordhaus, W. D. (1980b) . Thinking about Carbon Dioxide: Theoretical 

and Empirical Aspects of Optimal Control Strategies. Discussion 

Paper No. 565. Cowles Foundation, Yale University, New Haven, Conn. 
OECD Interfutures Project (1979) . Facing the Future. Organization for 

Economic Cooperation and Development, Paris. 
Raiffa, H. (1968) . Decision Analysis; Introductory Lectures on Choices 

under Uncertainty. Addison-Wesley, Reading, Mass. 
Reilly, J. , R. Dougher, and J. Edmonds (1982). Determinants of Global 

Energy Supply to the Year 2050. Contribution 82-6 to the Carbon 

Dioxide Assessment Program. Institute for Energy Analysis, Oak 

Ridge Associated Universities, Oak Ridge, Tenn. 
Ridker, R. G., and W. D. Watson (1980). To Choose A Future; Resource 

and Environmental Consequences of Alternative Growth Paths. Johns 

Hopkins U. Press, Baltimore, Md. 
Tversky, A., and D. Kahneman (1974). Judgement under uncertainty. 

Science 185:1124-1131. 



153 

Tversky, A. , and D. Kahneman (1981). The framing of decisions and the 

psychology of choice. Science 211:453-458. 
Uzawa, H. (1962) . Production functions with constant elasticities of 

substitution. Rev. Econ. Studies 29:291-299. 
Varian, H. (1980). Microeconomic Analysis. Norton, New York. 
Workshop on Alternative Energy Strategies (WAES) (1977) . Energy: 

Global Prospects 1985-2000. Report of the Workshop on Alternative 

Energy Strategies, McGraw-Hill, New York. 
Yohe, G. W. (1979) . Comparisons of price and quantity controls a 

survey. J. Comp. Econ. 3:213-234. 



2.2 A REVIEW OF ESTIMATES OF FUTURE CARBON DIOXIDE EMISSIONS 
Jesse H. Ausubel and William D. Nordhaus 

2.2.1 Introduction 

In analyzing prospects and policies concerning future carbon dioxide 
buildup, it is necessary to begin with projections of levels of C0 2 
emissions. Because of the long residence time in the atmosphere of 
C02 emissions, along with the potential for large and durable societal 
impacts of higher CO 2 concentrations, there is great interest in 
long-term projections those extending a half -century or more. While 
it is clearly necessary to make global long-term projections in this 
area, the projections are intrinsically uncertain, and the uncertainty 
compounds over time. 

This section reviews methods involved in making projections of 
carbon dioxide emissions, describes the major projections, and offers 
some comparisons and comments* It is intended to serve three 
purposes. First, it should help to acquaint the reader with the state 
of the art in C0 2 forecasting and the range of previous forecasts. 
Second, this review may help to identify shortcomings of current 
efforts and point to directions for new research. Third, it should 
establish the context of the forecasts developed by Nordhaus and Yohe 
(Section 2.1) for this report. 

Projections of future trajectories of C0 2 emissions can be roughly 
divided into three categories: (A) projections that are no more than 
extrapolations and that are primarily intended to be used to initiate 
studies of the carbon cycle or the climate system; (B) those based on 
relatively detailed examination of global energy supply and demand in 
which C0 2 emissions are largely incidental; (C) projections deriving 
from analysis of the energy system in which changing levels of CO 2 
are themselves taken into account. Leading examples of category A, in 
which C0 2 emissions are projected with little more than passing 
reference to energy modeling, are Keeling and Bacastow (1977) and 
Siegenthaler and Oeschger (1978) . These papers extrapolate emissions 



154 

in order to predict future atmospheric CO 2 levels. Such efforts also 
appear in numerous reports and papers concentrating on calculating 
climatic change, for example, JASON (1979) and Hansen et al. (1981). 
The projections consist of little more than extrapolating rates of 
fossil fuel emissions growth from recent decades out a century and more 
into the future. These extrapolations can be regarded as simplifica- 
tions or summarizations of more complete projections; they are useful 
for studies of the sensitivity of the carbon cycle and climate system 
but unpersuasive as elements of a comprehensive C0 2 assessment. 

The projections based on relatively detailed analysis of an uncon- 
trolled global energy-climate system, (B) r which are the most important 
for purposes of this section, differ greatly in their design, in the 
extent to which formal models are employed, and in detail with respect 
to fuels, geography, and other factors. Leading examples include those 
made by H. Perry and H. H. Landsberg (1977) for the NRC Geophysics Study 
Committee, the several projections by Rotty and by Edmonds and Reilly 
of the Institute for Energy Analysis (IEA) of Oak Ridge Associated 
Universities (Rotty, 1977; Rotty and Marland, 1980; Edmonds and Reilly, 
1983a) , the projections of Nordhaus (1977) and (1979) , and those made 
for the Energy Systems Program of the International Institute for 
Applied Systems Analysis (IIASA) (Niehaus and Williams, 1979; IIASA, 
1981) . 

Category (C) projections, which require the basic analysis of 
category (B) as input, seek additionally to take into account the 
changing level of atmospheric C0 2 (or the costs of climatic change) 
in the calculations. That is, C0 2 is included as a possible eventual 
constraint pn the energy system. Projections incorporating this per- 
spective are found in Nordhaus (1979, 1980), Council on Environmental 
Quality (CEQ) (1980), A. M. Perry (1982), Perry et al. (1982), and 
Edmonds and Reilly (1983a) . 

Almost all of the scenarios applied to studies of C0 2 that are 
based on reasonably in-depth analysis of the energy situation project a 
continued growth of energy demand (or consumption) to between about 20 
and 40 terawatt (TW) years per year (yr/yr) over the next 40 or 50 
years, an increase of two and a half to five times the recent level.* 
These include scenarios developed for studies by the National Research 
Council (NRC) , the International Institute for Applied Systems Analysis 
(IIASA) , and the Institute for Energy Analysis (IEA) of Oak Ridge 
Associated Universities and by Nordhaus (1979) . Several other energy 
scenarios, like those of the Interfutures Project (1979) , the Hudson 
Institute (Kahn et al., 1976), the World Energy Conference (1978), and 
Stewart (1981) are in the same range. Whenever such scenarios do not 
project a large share of nonfossil energy, they lead to relatively 
serious concerns about climatic change in the next 50 to 100 years.t 



*Estimated global primary energy supply in 1975 was roughly 8 TW 
yr/yr (IIASA, 1981) . 

tThe market share of nonfossil energy sources (including noncom- 
mercial energy) is about 15% at present. The prominent scenarios 

(continued on facing page) 



155 

Most of the estimates of C0 2 emissions from fossil fuels in the year 
2030 lie in a range between about 10 and 30 gigatons of carbon (Gt of 
C) . Thus, based on a review of past projections/ it appears that the 
range of estimates of energy consumption 50 years hence is a factor of 
2 or more, and consequent CO 2 emissions show a range of a factor of 3 
or more. 

It should become clear that the range of estimates is wider than the 
range of approaches. The large differences in the estimates are trace- 
able in almost all cases to the sensitivity of the models to differences 
in estimates of the variables or parameters. Most prominent are assump- 
tions about rates of population growth, economic growth, the ratio of 
energy demand to economic activity, and the mix of supply sources that 
will meet energy demand. Brief descriptions of the major projections 
in the three categories follow. 



2.2.2 Projections Based on Extrapolations 

The extrapolative (A) models are essentially one-equation global models. 
There are no nations, no economic sectors, no GNP or population projec- 
tions. In these models, an idealized resource depletion function is 
customarily used to project the evolution of annual releases through 
future centuries. There are usually three key variables in the func- 
tion. One is the total resource of carbon-based fuels. The second is 
the initial growth rate. The third is a parameter that embodies judg- 
ments about the future pattern of exploitation of the resource. It can 
be set such that peak exploitation occurs when the resource is, for 
example, 20% depleted, with the possible intention of reflecting a con- 
sumer response to rising prices. Or, it can be set to draw different 
patterns of exploitation, for example, short and intensive, or gradual. 

In the very long run (past 2100) , the key variable determining C0 2 
buildup in this approach is the total carbon resource. The studies 
have typically taken a number in the vicinity of 5000 Gt of C for the 
total carbon resource. Such a figure is not out of line with estimates 
of ultimately available resources, although it is a factor of 10 larger 
than today's proved recoverable reserves (see World Energy Conference/ 
1980). 

In the medium run (up to 2100) , the central variable determining the 
C0 2 buildup in simple extrapolation models is the initial growth 
rate. It has been common in the literature to base this variable on 
work of Rotty (1977) , who estimated from historical data that CO 2 
emissions from fossil fuel burning (with a trivial addition for cement 
manufacture) increased 4.3% per year if one excludes the periods of the 
two world wars and the global economic depression of the early 1930s. 
This figure of 4.3% has been extremely influential and has been widely 
used to project future levels of atmospheric CO 2 , Many papers and 

mentioned above generally foresee either an unchanged share for 
nonfossil sources or a moderate expansion of nonfossil sources, to 
about 20-35% over the next 50 years. 



156 

reports on CO2-induced climatic change written in the past few years 
mention it prominently. For example, a JASON report (1979) opens with 
the statement, "If the current growth rate in the use of fossil fuels 
continues at 4.3% per year, then the C0 2 concentration in the atmo- 
sphere can be expected to double by about 2035. . . . " 

While the 4.3% figure is the one most mentioned in the climate 
literature, increasing debate has grown around it (e.g., World Climate 
Programme, 1981) . One reason for the recent skepticism is that energy 
growth has slowed considerably to an average of little more than 2% 
annually since 1973. Projections reviewed below range from that of 
Lovins (1980; Lovins et al., 1982), who suggests there might be a 
global decrease in use of energy and fossil fuels, to the 50 TW yr/yr, 
case proposed by Niehaus and Williams (1979) , in which energy demand 
grows at an average of about 4% in coming decades and all of this high 
projected energy demand is covered by fossil fuels. 

It is worth noting that the highest projections of C0 2 emissions 
have generally come from the simple extrapolative models and rarely 
from studies that incorporate explicit supply and demand models for 
energy. To illustrate, the Keeling and Bacastow (1977) "preferred 
scenario" projects emissions somewhat larger than the high coal 
scenario developed by Niehaus (1979) as an upper limit scenario from 
the energy perspective, and the Siegenthaler and Oeschger (1978) 
"upper-limit" scenario generates emissions at about twice the rate of 
the Niehaus high scenario. 

The extrapolations might be best characterized as "gedanken experi- 
ments" devised for study of the carbon cycle: "suppose x thousand tons 
of carbon exist as fossil fuels and all will be used at a certain 
rate. ..." While extrapolative models have been successful in drawing 
attention to the C0 2 issue, they are of limited interest in projecting 
likely outcomes for C0 2 emissions and concentrations. The main virtue 
of the approach is simplicity, for a constant growth or logistic curve 
has great transparency, particularly relative to the enormously com- 
plicated energy models. On the other hand, these models do not respect 
fundamental aspects of economic and energy sector behavior, such as 
conservation based on rising energy prices. It is not surprising that 
these models will, therefore, lack realism during periods (after 1973, 
for example) when changes in economic and political structures have 
been profound. 



2.2.3 Energy System Projections 

The major class of forecasts of C0 2 emissions arises from formal or 
informal energy modeling. Most of this work dates from the 1973 "energy 
crisis" and is only recently published. In general, it forms the most 
reliable basis on which to draw for projections. Note that only global 
studies are sufficient for projecting C0 2 emissions; the numerous 
national and regional energy studies may provide a consistency check on 
global studies, but they cannot be used independently to project C0 2 
emissions. 



157 
2.2.3,1 Perry-Landsberg (WAS) 

Perry and Landsberg (1977) assembled projections of world energy con- 
sumption and emissions to the year 2025 for the NAS report f Energy and 
Climate. The projections are for 11 geographic regions , which are 
sometimes large nations and sometimes groups of nations. Regional 
demand for energy is derived from projections of population f GNP, and 
the relationship of GNP per capita and energy consumption. A "high- 
population/low-economic-growth" situation is postulated for developing 
countries and a n low-populat ion/low-economic-growth 11 situation for 
developed countries. Global population in 2025 is at 9.3 billion, 
about 20% higher than IIASA and Rotty. The net result is a total 
energy demand forecasted to reach about 39 TW yr/yr in 2025. 

Emissions are calculated for two situations chosen to stress the 
contrast between a strategy based on "renewables" (i.e./ noncarbon- 
based, abundant energy sources) and one based on coal. In the first 
case, if regional demand exceeds regional production, an estimate is 
made assuming the new noncarbon-based energy resource is available to 
meet the deficiency of nonrenewable resources. In the second case, an 
estimate is made for the situation for which regional deficiency would 
be met by coal. Based on these assumptions, annual world CO2 
emissions in 2025 would be between 13 and 14 Gt of C in the first case 
and about 27 Gt of C in the second, or about 2.5 to 5 times current 
levels . 

The Perry-Landsberg study forms a careful baseline for comparison. 
It is comprehensible and plausible. A major shortcoming is that it 
omits any explicit role for prices to play in driving demand toward or 
away from energy in general or individual fuels in particular. In addi- 
tion, while the total demand for energy grows out of a well-specified 
model, the fuel mix is based on arbitrary assumptions. 



2.2.3.2 IIASA 

2.2.3.2.1 Niehaus and Williams 

The IIASA Energy Systems Program analyzed several hypothetical 
energy strategies for the period up to the year 2100 for their 
implications for atmospheric 002 (Niehaus and Williams, 1979; IIASA, 
1981) . As such, it could not directly employ the so-called "IIASA 
energy models," which were run only to the year 2030. Rather, distri- 
bution of energy supply among coal, oil, gas, solar, and nuclear is 
derived from a very-long-term energy model developed by Voss (1977) . 
The Voss model employs principles similar to that of the Forrester- 
Meadows (system dynamics) school and is structured into six sectors: 
population, energy, resources, industrial production, capital, and the 
environment. It is global; there is no geographic disaggregation. 

Among the strategies explored (Niehaus and Williams, 1979) are four 
in which global demand levels out to either 30 TW yr/yr or 50 TW yr/yr 
in the mid-twenty-first century and remains at that level to 2100. In 
both the lower- and higher-demand cases there is an analysis in which 



158 

nuclear and solar energy play an important role and in which they do 
not. Table 2.19 shows the reserves of fossil fuels used in each 
strategy. The relation between total coal use and CO 2 emissions is 
characteristic of projections leading to high or low CO 2 emissions. 

The scenarios with reliance on nuclear and solar energy lead to peak 
annual C0 2 emissions of about 8 to 10 Gt of C around the year 2000, 
while the scenarios with reliance on fossil fuels lead to emissions of 
about 22 and 30 Gt of C in 2030, increasing somewhat thereafter. While 
consideration is given to available fossil resources at the global 
level, there is no study of regional or national implications. 

The Niehaus-Williams projections are based fundamentally on judgments 
external to economic analysis or modeling. Once the energy growth path 
and fuel mix are set, the outcome for C0 2 emissions is determined 
the use of the system model plays but a small role in the outcome. 

The most important issue is whether the ex cathedra judgments as to 
the ultimate levels of global energy demand (the 30 and 50 TW yr/yr 
levels discussed above) are reasonable. While such figures are 
conventional, and indeed so often used that they become comfortable 
assumptions, they have no grounding in a physical or economic con- 
straint or in the outcome of an energy model. The notion of "satura- 
tion" at these levels is a popular idea that has no particular basis, 
other than the hope that human society will pass through a transition 
to a stable plateau over the next couple of generations. Thus, while 
the critical assumptions of energy demand and fuel mix in these studies 
do not appear implausible, their grounding is weak. 



2.2.3.2.2 The I I AS A Energy Models 

IIASA used a set of extremely detailed models to delineate two 
scenarios, a "high" and a "low" case culminating in 2030 with world 
energy consumption at 35 and 22 TW yr/yr, respectively. The models are 
oriented toward engineering and technical considerations for specific 
demand sectors and global consistency of supply among the seven regions 

TABLE 2.19 Reserves of Fossil Fuels Used in Different Outcomes 
(1975-2100)* 



Coal Oil Gas 

Strategy (Gt of C) (Gt of C) (Gt of C) 

30 TW with solar and nuclear 170 170 110 

50 TW with solar and nuclear 230 210 130 

30-TW fossil fuel 1980 190 120 

50-TW fossil fuel 3020 230 140 



SAf ter Niehaus and Williams (1979) . 



159 

into which the world is disaggregated. The distribution of supply 
sources in the actual 1 1 AS A scenarios is quite different from Niehaus 
and Williams, even though the Niehaus and Williams runs were originally 
chosen to be broadly consistent with the global energy demand pattern. 
Both the high and low 1 1 AS A scenarios are hybrids, with expanded use of 
many supply sources/ so that in 2030 11 TW yr/yr are coming from non- 
fossil fuel sources in the high case and 7 TW yr/yr in the low case. 
In the high case emissions are above 16 Gt of C in 2030 f and in the low 
case they are near ing 10 Gt of C. 

The IIASA models have probably been the closest existing approach to 
an appropriate disaggregated technique for forecasting CO 2 emissions. 
In principle, they are grounded in engineering and economic relations, 
with attention to feasibility and response of supply and demand to 
price. In practice, because of the need to accommodate differing views, 
world energy consumption was adjusted judgmentally to be "reasonable," 
as well as on the basis of the formal methods. In this respect, the 
outcome shares the problems outlined in the last paragraph of the dis- 
cussion of the Niehaus-Williams approach. 

Another issue raised by the IIASA model is whether a high degree of 
disaggregation is appropriate. Such an approach allows considerations 
such as those involving trade and national policies; however, it also 
makes the models difficult to comprehend, manipulate, change, and 
verify independently. 



2.2.3.3 Rotty et al. 

For several years, Rotty and co-workers at the Institute for Energy 
Analysis of Oak Ridge Associated Universities emphasized extrapolation 
of the 4.3% estimate of historic annual increase in CO 2 emissions and 
figures tapering off from this (Rotty, 1977, 1978, 1979a f b; Mar land and 
Rotty, 1979) . Based on demand and fuel-share projections made for six 
world regions, an annual fossil fuel release of C02 containing 23-26 
Gt of C from energy use of 36-40 TW yr/yr in the year 2025 is calcu- 
lated. The work is partly based on a more formal analysis by Allen et 
al. (1981) developed for the year 2000. In extension of the projections 
to 2025 Rotty assumes supply will meet demand without examination of 
balancing economic factors such as prices. For projections of emissions 
beyond 2025, a different extrapolative technique involving application 
of arbitrary global fossil resource depletion rates is employed (see 
Section 2.2.3.2 above). 

A more recent paper (Rotty and Mar land, 1980) includes some discus- 
sion of constraints on fossil fuel use. Three kinds of constraints are 
examined: resource, environmental, and fuel demand. With respect to 
resource supplies, Rotty and Marland conclude that "the fraction of 
total resources used up to the present is so small that physical quan- 
tities cannot yet be perceived as presenting a real constraint. 11 
However, it is mentioned that unequal geographic distribution of the 
resources probably will continue to be a source of international stress. 
Climatic change as an environmental issue is dismissed as a constraint 
to fossil fuel use. 



160 

In contrast, Rotty and Mar land (1980) discuss at some length the 
likelihood that slower growth in fuel demand dictated by social and 
economic factors will limit fossil fuel use. Reduced rates of economic 
growth are projected as a result of very recent trends and anticipated 
problems with capital and escalating costs and shifts toward conserva- 
tion and less energy intensive industries. No formal modeling is 
offered to substantiate the position. Regardless of the precise causes/ 
summing up estimates for about a dozen countries and half a dozen com- 
posite regions leads Rotty and Marland to project (#2 emissions in 
2025 of about 14 Gt of C in a 26 TW yr/yr global energy scenario an 
annual growth rate of 2% per year from today. Thus, the range of Rotty 
emission projections are quite similar to those of IIASA and Perry and 
Landsberg . 

The strengths and weaknesses of the Rotty approach are partly those 
of the extrapolative models and partly those of the Perry-Landsberg 
approach. The repeated adjustment of assumptions is evidence of the 
uncomfortably arbitrary nature of the endeavor. 



2.2.3.4 Nor dhaus 

Nordhaus (1977 , 1979) estimated the uncontrolled path of CO^ emissions 
in a modification of a model developed for studies of efficient alloca- 
tion of energy resources (or of a competitive market for energy) . 
Nordhaus f s approach was fundamentally based on economic modeling and 
assumptions with interaction of forces of supply and demand leading to 
a path of prices and energy consumption over time. By comparison f 
prices play a lesser role in the IIASA and Perry and Landsberg 
approaches and virtually no role in Rotty 's projections. 

Nordhaus employs a medium-sized linear programming model, with basic 
components being an objective function (based on demand functions for 
energy) and a supply function centered on geological considerations and 
technology. The outcome is calculated by finding the lowest cost way 
of meeting the demand function, using a linear programming (LP) algo- 
rithm. The demand function (technically, the objective function) for 
the LP is drawn from data on market behavior. It is built up from four 
energy sectors (electricity, industry, residential, and transportation) , 
with demand in each sector a function of population, per capita income, 
and relative prices. The technology or constraint set is derived from 
engineering and geological data on resource availability and costs of 
extraction, transportation, and conversion. The model incorporates 
constraints on new technologies, adaptation of demands, and upper bounds 
on rates of growth. 

Running the model involves balancing supply and demand over time, 
with prices playing the central role of equilibration. Results are 
given in terms of both activity levels (for example, production of coal 
or oil in a given period) and prices. The calculation provides for six 
different fuels used in the four energy sectors, for two different 
regions (United States and rest of world) , for ten time periods of 20 
years each. The macroeconomic assumptions are that rapid growth in GNp 
per capita will continue in both regions, but at a diminishing rate 



161 

after 2000, and that population will also slow to reach a world level 
of 10 billion in 2050. 

Nordhaus calculates that the uncontrolled path leads to large changes 
in the level of atmospheric CO 2 . In the uncontrolled case, annual 
emissions are at 18 Gt of C in 2020 and steeply increasing, so they 
reach 40 Gt of C in 2040. Global energy demand is about 40 TW yr/yr in 
2030. A key in these high projections is high initial GNP growth rates? 
for 1975-1990 the assumed growth per year is 3.7% in the United States 
and 6.5% in the rest of the world. 

The results of the Nordhaus analysis exhibit the strengths and weak- 
nesses of pure (non judgment-based) economic modeling. On the one hand, 
the outcome is based on objective data (such as market prices and 
resource availability) and is thus reproducible and can be easily 
modified over time. On the other hand, results are very sensitive to 
assumptions about future price and growth trends. The actual model 
runs were based on an assumption of rapid future economic growth and 
low fuel prices leading thereby to rapid estimated growth of CO 2 
emissions and atmospheric concentrations. 



2.2.3.5 Edmonds and Re illy 

Rotty's work is now being followed at IEA by a more formal CO 2 
emissions model developed by Edmonds and Reilly. The model takes as 
inputs key economic, resource availability, and demographic variables 
such as income, energy costs, resource constraints, labor force, and 
population. From these it calculates consistent energy-use paths. 
Consistency is defined as a balancing of supply and demand in the face 
of resource constraints, with energy prices adjusting to assure an 
equilibrium solution. Energy use is disaggregated into nine world 
regions and all major possible fuel types, including oil, gas, coal, 
coal liquefaction, and shale oil. The model is intended to be 
applicable out about 100 years, with calculations feasible at intervals 
that the user selects, for example, 10 or 20 years. The model is 
extensively documented (Edmonds et al., 1981; Reilly et al., 1981? 
Edmonds and Reilly, 1983c) , and a base case is now being developed 
(Edmonds and Reilly, 1983a,b) . 

Initial results show a quite steady increase in energy demand of 
about 2.5% per year from now to 2050, so that demand has reached about 
29 TW yr/yr in 2025 and 50 TW yr/yr in 2050. C0 2 emissions increase 
by 1.5% per year from now to 2000, and by 2.3% per year between 2000 
and 2025, reaching 12 Gt. Because of increasing reliance on coal, oil 
shale, and synthetic fuels, emissions then rise quite steeply, by more 
than 3% per year, and reach an annual rate of 26 Gt of C by 2050* 

The Edmonds-Re illy model has the potential of being an extremely 
useful follow-up to earlier detailed studies, such as that of I I AS A. 
It contains sufficient regional and sectoral disaggregation that 
experts in individual areas (such as analysts specializing only in the 
U. S. economy or a particular fuel source) can evaluate the detailed 
forecasts and assumptions. It also appears to be flexibly designed, so 
that results of different assumptions can be examined easily. 



162 

At the same time, the current effort contains some of the problems 
that have plagued earlier large-scale energy models. Perhaps the most 
important is the decoupling of energy demand from output. The current 
model has energy demand sensitive to prices and incomes, but incomes 
and outputs are not directly related to energy, labor, and other inputs. 
(Technically, energy is not treated as a derived demand, that is, 
derived from a production function relating inputs to output.) A 
second problematic feature is the extensive use of logistic curves that 
are not sensitive to prices for determining supply. 

It should also be noted that the Edmonds-Reilly model is quite large 
and somewhat forbidding for a casual user* The benefit from techno- 
logical and regional detail is partially vitiated by the difficulty of 
understanding the structure and workings of the model. As in many 
large-scale models, the size makes identification of critical parameters 
or assumptions a formidable task. 

Notwithstanding these reservations, the Edmonds-Reilly work stands 
out today as the only carefully documented long-run global energy model 
operating in the United States. 



2.2.3.6 Other Projections 

Marchetti (1980) has made a forecast of the amount of C0 2 that will 
be emitted to the year 2050 based on a logistic substitution model of 
energy systems (Marchetti and Nakicenovic, 1979). This model treats 
energy sources as technologies competing for a market and applies a 
form of market penetration analysis. A logistic function is used for 
describing the evolution of energy sources and is fitted to historical 
statistical data. The driving force for change in this model appears 
to be the geographical density of energy consumption, and the mechanisms 
leading to the switch from one source to another are the different tech- 
nical characteristics associated with each energy source. For example, 
in the Marchetti view oil succeeded coal primarily because of the advan- 
tages achievable by a system operating on fluids. 

With data on energy consumption back to 1860 and including both com- 
mercial and noncommercial (wood, farm waste, hay) energy sources, the 
slope of the fitted curve of energy demand implies an annual growth of 
2.3%. [This contrasts strongly with Rotty (1979b) , who emphasizes that 
commercial energy supply, excluding times of world conflicts and depres- 
sion, has grown at a rate of about 5.3% since I860.] Applying a future 
growth rate of 3% per year, Marchetti calculates energy consumption for 
the various sources for the period 1975-2050 based on the logistic equa- 
tions. The model predicts a relatively rapid phaseout of coal, a 
dominant role for natural gas, rapid growth of nuclear power, and a 
negligible role for new sources other than nuclear over the next 50 
years. The model implies an increase in annual C02 emissions to about 
14 Gt of C in 2030, an amount close to the lower estimates of Perry and 
Landsberg, IIASA, and IEA, and a cumulative emission of carbon to the 
atmosphere between the years 1975 and 2050 of about 400 Gt of C [to 
somewhat less than 450 ppm(v)]. Perhaps more important, it predicts a 
gradual reduction in emissions and atmospheric C(>2 thereafter, rather 
than continuing increase. 



163 

While Marchetti's projection of fuel shares is singular , his analysis 
of the long-term pattern of energy demand is not. Stewart (1981) also 
uses an empirical approach leaning on application of logistic growth 
curves r chosen to fit historical data extending back to 1850. Stewart 
argues additionally that energy growth is likely to evolve in surges or 
cycles rather than monotonically. Stewart identifies historical 
"cycles" in energy use with periods of around 50 years (perhaps a 
manifestation of the frequently cited Kondratieff cycle of economic 
activity) and notes that deviations of plus or minus 20% around a 
long-term logistic growth curve were experienced. 

On the basis of an assumed stable cyclical structure, Stewart pro- 
jects world energy consumption to the year 2025. For the period 
1975-2000 a 40% growth is indicated; this breaks down into zero energy 
growth in the United States and a 60% growth for the world outside the 
United States. This overall projection for 2000 is lower than most. 
However , Stewart's projection for 2025 is fclose to other high values. 
After the relatively depressed period between 1975 and 2000 , world 
energy growth between 2000 and 2025 is projected at a rate of about 4% r 
increasing from about 13 TW yr/yr to almost 36 TW yr/yr. 

Legasov and Kuz'min of the Atomic Energy Institute of the USSR have 
also made a projection employing a logistic approach. The key variable 
in their function is one that describes the level of stabilization of 
per capita energy consumption (Legasov and Kuz'min, 1981; Report of the 
US/USSR Workshop, 1982). Legasov and Kuz'min explore two cases, one in 
which global average annual per capita energy consumption by 2100 
reaches 10 kW (roughly the level in the United States today) and one in 
which it reaches 20 kW. Population, meanwhile, stabilizes at a level 
of 12 billion people. Under these assumptions global energy use in 2020 
is either 50 or 60 TW yr/yr, with a population of 8.8 billion. Legasov 
and Kuz'min project coal and nuclear power as the principal energy 
sources for the coming decades, with nuclear power gradually becoming 
dominant. Under these assumptions, CO 2 emissions in 2020 are about 
15 Gt in the lower case and 18 Gt in the upper case and roughly stable 
for several subsequent decades. 

The three approaches described above are more sophisticated than the 
extrapolation approach (see Section 2.2.3.2), but the underlying method- 
ology is similar. All assume that there is a stable underlying dynamic 
(exponential, logistic, or logistic-cum-sinusoidal) and forecast off 
that base. These approaches allow for no structural relation between 
exogenous variables like population and resources and endogenous vari- 
ables like energy consumption. Such autoregressive or inertial models 
do relatively well at prediction in the short run, but their level of 
aggregation is so high that for most purposes one must still turn to 
the more structural models. 

A final source is Lovins (Lovins, 1980; Lovins et al., 1982), who 
projects very low C02 emissions because of a shift to conservation 
and renewable (nonfossil) sources. With a 4.6-fold increase in global 
economic activity during 1975-2080 and a doubling of world population, 
total energy needs will, according to Lovins, be below the 1975 level, 
indeed dropping over the next century to less than half the present 
level, A projected increase in energy efficiency in end uses along 



164 

with renewable sources for energy production might, according to Lovins f 
largely or wholly eliminate the global use of fossil fuels. A case 
study of the Federal Republic of Germany, a diverse heavily industrial- 
ized economy in a rigorous climate, is used as an "existence proof 11 
(basis for extrapolation) for the efficiency and renewables strategy. 

Levins 's results appear to be wishful with respect both to rapid 
development and diffusion of solar technologies and to lifestyle changes 
involving energy conservation. He does not present a formal model or 
develop the implications of the analysis for capital and labor needs. 
In addition, some of the trends identified r such as increasing efficien- 
cy of end-use devices, may raise the demand for energy, an outcome not 
accounted for. While Lovins may turn out to be correct, the analytical 
basis for his views remains elusive and characterized by strong cultural 
bias. 



2.2.4 Projections with CO^ Feedback to the Energy System 

The energy projections reviewed in Section 2.2.3 share a potential 
deficiency when used to generate long-term CO? emission trajectories. 
Calculation of the ejected C0 2 is largely incidental. An energy path 
is plotted for a variety of reasons, and C0 2 is merely the outcome of 
the chosen path. 

There are two ways in which such an approach may be deficient. 
First, by focusing on C0 2 directly, it may be possible to get more 
accurate CO 2 forecasts, as secondary issues (such as coal versus oil) 
can be ignored. Second, if a C0 2 buildup takes place and leads to 
serious social consequences, there may be some impact on the economy 
directly (through output) or indirectly (through policy reactions) . 
Put differently, models that allow very high C0 2 but do not allow 
feedback from environmental change to energy policy must be regarded 
with caution; they mask significant assumptions about the behavior of 
people and governments (Stahl and Ausubel/ 1981). 

Projections that include increased CO 2 levels as a possible 
eventual constraint on C0 2 emissions include Nordhaus (1979, 1980) , 
Edmonds and Reilly (1983a,b) , CEQ (1980) , and Perry (1981; Perry et 
al., 1982). These projections generally require that some threshold 
concentration of C0 2 (or similar constraint) be set, presumably by 
political intervention. Trajectories are then calculated that keep 
ambient levels from exceeding this threshold. Thus, in these 
approaches, rather than begin from high- and low-energy scenarios, the 
approach is to work backward from a desired or specified terminal 
condition to defining energy demand and fuel mix patterns that satisfy 
it. 



2.2.4.1 Nordhaus 

Along with estimating the uncontrolled path described earlier, Nordhaus 
(1977, 1979) also estimates time paths of emissions given particular 
carbon dioxide constraints. Efficient allocation of energy resources 



165 

is calculated using the model described earlier under the assumption 
that it would be necessary to prevent atmospheric CO 2 from exceeding 
either 1.5, 2, or 3 times the preindustrial level [about 450 r 600 f or 
900 ppm(v)] . 

The optimal path does not differ from the uncontrolled path for the 
first period (up to 1990) . Abatement measures become necessary only in 
the second period (1990-2010) for the stringent control (450 ppm) and 
in the third period (2010-2030) for the milder control programs. To 
illustrate, in 2020 emissions for the uncontrolled case and the trip- 
ling are identical at 18 Gt of C, and the doubling case is only mar- 
ginally lower at 16 Gt of C, but the 50% increase limit requires 
emissions of only 4 Gt of C. In 2040 the stringent-case emissions have 
trailed off to barely more than 2 Gt of C, the doubling case leaves 
carbon emission steady at 16 Gt of C, while the tripling and uncon- 
trolled case have both reached the vicinity of 40 Gt of C per year. 
This technique allows estimates of the costs of controlling CO 2 
emissions as well as the "carbon taxes" necessary to induce such 
responses. 

Nordhaus (1980, 1982) also develops an optimal control framework, 
which seeks to identify the most economical way to balance the exploita- 
tion of both carbon fuels and climatic resources. The analysis is at a 
highly aggregate, global level; implications for sectors or for regional 
or national policies are not explored. Nordhaus weighs CO 2 control 
strategies according to two criteria: their effects on the paths of 
consumption that are generated by the control strategy and maximization 
of the discounted value of consumption streams, where the discount rate 
combines both a temporal and a growth factor. 

The framework consists of four simple equations. These are a 
description of the carbon cycle and climatic effects of COn eleva- 
tion, estimates of the costs of reducing or abating C0 2 emissions, an 
equation that incorporates estimates of economic impacts of CO 2 
buildup, and an equation that represents inter temporal choice between 
consumption paths. 

Since there is great uncertainty about the economic and social impact 
of elevation of C0 2 concentration, Nordhaus tests the sensitivity of 
the model to different sets of costs. These are described by a "loss 
parameter, 11 which indicates the fractional loss of consumption per 
doubling of CO 2 . By varying this and other parameters, a set of 
emissions trajectories is calculated. The outcome of the model was 
considered at best illustrative given the uncertainty surrounding key 
parameters (such as the economic impact of climate change) . A major 
result, however, was that the best degree of CO 2 control was 
extremely sensitive to important uncertain parameters, that is, no 
obvious control strategy stood out. 

2.2.4.2 Edmonds and Re illy 

Edmonds and Reilly (1983a,b) have also begun to explore the effect of 
taxation policies of various kinds on CO 2 buildup. One question asked 
is what consequences a substantial CO 2 tax in the United States would 



166 

have on the level of atmospheric C0 2 . They find that global carbon 
emissions are reduced by much less than the U.S. reduction owing to the 
fact that decreased U.S. energy demand resulting from the C02 tax 
lowers world energy prices, which in turn spurs energy consumption in 
other regions. In contrast, when a global tax is combined with a U.S. 
embargo on coal exports, there are substantial reductions in U.S. and 
non-U.S. CC>2 emissions. 

While all the studies on taxation of C0 2 are still quite tentative, 
the three sets of tax experiments that we have reviewed the Nordhaus 
results discussed in Section 2.2.4.1, the Nordhaus-Yohe results in 
Section 2.1, and the Edmonds-Re illy results appear broadly consistent. 
This finding is striking, given that the three approaches are very 
different. 



2.2.4.3 CEQ 

The CEQ study (1981) derives several curves to yield a buildup of 
atmospheric C0 2 equal to 1.5, 2.0, and 3.0 times the pre industrial 
level (slightly less than 450, 600, and 900 ppm, respectively). It 
employs a simple, two-equation global model consisting of a differ- 
ential equation to explain buildup of atmospheric CO 2 and a logistic 
equation to forecast C0 2 emissions. The major unknown parameter is 
the initial growth rate of fossil fuel combustion. The model is run 
backward to calculate global fossil fuel releases that would produce 
the assumed buildups of carbon dioxide. Curves preferred for further 
analysis correspond in 2030 to fossil fuel energy production of about 
8, 13, and 17 TW yr/yr and emissions of 6, 10, and 13 Gt of C, 
respectively. 

The controlled curves are compared with two overall energy projec- 
tions. The high global energy demand scenario is for a world whose 
population has leveled off at 10 billion by the year 2100 and an aver- 
age per capita energy use equal to two thirds the present U.S. level. 
Energy use in 2030 is about 35 TW yr/yr (similar to the IIASA high 
scenario) and rises to about 75 TW yr/yr by 2100, a ninefold increase 
over current levels, with about one fourth accounted for by population 
growth. A lower world energy use scenario represents a world whose 
population has leveled off at about 8.5 billion by 2100 at an average 
per capita level of one third present U.S. consumption. In 2030 energy 
use is about 20 TW yr/yr (similar to the IIASA low scenario) and reaches 
a plateau well before 2100 of about 30 TW yr/yr, about a fourfold 
increase over current consumption in which one half the growth is 
attributable to population increase. 

The CEQ study evaluates the significance of the gap between overall 
energy demand and the three assumed CO 2 limits. For example, to 
avoid exceeding a 50% increase in global CO 2 concentration and to 
meet the low-energy-demand scenario (a low growth, environmentally 
cautious world) , nonfossil fuel sources would be required to increase 
from about 1 TW yr/yr today to more than 4 TW yr/yr by the year 2000, 
or about a 9% growth per year. By 2020 nonfossil fuel sources would 
have to contribute about 10 TW yr/yr, with their growth averaging 4% 



167 

between 2000-2020. The 10 TW yr/yr are more than the current total 
global annual energy use r and more than the nonfossil (solar and 
nuclear) supply estimated for 2020 in the IIASA high scenario. The CEQ 
study estimates that together hydropower and nuclear power could 
probably provide between 2 and 3 TW yr/yr (fuel equivalent) by the year 
2000. Thus, with low-energy growth but also a low ceiling on CO 2 
levels, a contribution of about 1 to 2 TW yr/yr would be needed by 2000 
from other renewables, with rapid increases thereafter as hydropower 
potential is exhausted. 



2.2.4.4 A. M. Perry et al. 

Perry and colleagues (1982; Perry et al. r 1982) begin by adopting global 
energy projections from IIASA, the World Coal Study (1980) f and others 
as reference scenarios. The novel parameter in their analysis is the 
date when global fossil energy use must begin to deviate from the 
reference scenarios in order to meet various atmospheric CO2 limits. 
This date is referred to as the action initiation time (AIT) . All the 
analyses so far are at the global level; that is, they refer to when a 
global policy (national policies summing to a global policy) would need 
to begin to be followed. 

The approach built around action initiation times stresses that the 
rate of change of energy strategies is extremely important. If some 
C0 2 limit were approached along the kinds of curves normally drawn, 
the limit would certainly be passed, because of the inertia or momentum 
of the energy system. If the ceiling were not to be exceeded, CO2 
production would have to fall abruptly to zero, a virtual impossibility. 
Thus, Perry (1982) proposes anticipatory scenarios, which involve a 
gradual slowing of growth of fossil fuel use, followed by an eventual 
slow decline. With a high initial growth rate or a late AIT, the 
transition required in order to remain below a given CC>2 target may 
be too rapid and the subsequent decline too steep the required 
transition may be infeasible. 

According to arbitrary feasibility criteria relating to historic 
evolution and behavior of energy systems, several scenarios are drawn 
that should allow sufficient time for the necessary changes in energy 
demand patterns and supply technologies. Table 2.20 lists some AITs 
thought to be of intermediate difficulty. 

In the Perry study, as well as the CEQ study, it is apparent that by 
fixing only a few parameters, principally energy growth, CO 2 limits, 
and a few characteristic times like market penetration/ the overall 
trends of fossil and nonfossil energy use become approximately deter- 
mined. With further work it may be possible to judge more reliably 
whether the different patterns of energy use designed to limit CO 2 
concentrations would be easy or difficult to attain. Without such 
information, it seems premature to employ these models for prescriptive 
purposes. 



168 
TABLE 2.20 Required Action Initiation Times for Various CO 2 



C0 2 Limit Initial Growth Rate of Annual Carbon Emissions 
(ppra) 1.5%/yr 2.5%/yr 3%/yr 



500 


2005 


1995 


1990 


600 


2025 


2010 


2000 


700 


2040 


2025 


2010 


800 




2035 


2020 



^Source: Perry (1982). 

For example f if a global limit for CO2 in the atmosphere of 500 

ppm is to be met, and emissions are growing at the outset by 1.5%/year/ 

actions to reduce the share of fossil fuels would need to begin in the 

year 2005. If emissions are growing by 3% in the coming decade and we 

wish to meet a limit of 500 ppm, policies to discourage use of fossil 

fuels might need to become effective as early as 1990. These action 

initiation times are for transitions away from fossil fuels judged by 

Perry to be of intermediate difficulty. 



2.2.4.5 General Comments 

Studies that attempt to include feedback from C0 2 concentrations to 
energy policy are in their infancy. A particular problem is the con- 
fusion and combination of "positive" and "normative" approaches. In a 
"positive" model, the attempt is to describe how a system will behave 
under given boundary conditions. In a normative approach, one sets up 
a policy goal or objective function and then asks how the system ought 
to behave in order to optimize the objective function. While the dis- 
tinction is seldom clearly delineated in global energy models (see 
particularly the comment on Lovins above) , potential confusion is most 
likely to arise concerning the class of models discussed in this 
section. For the most part, the best interpretation would seem to be 
the following: the energy systems are based on a positive description, 
and C0 2 constraints are viewed as alternative normative policy 
constraints. However, assumptions of inaction (or absence of feedback) 
at very high levels of C0 2 emissions are also in a sense normative. 

A second issue concerns the actual limits imposed. While the 
limitation of a doubling of CO 2 is the policy most often analyzed, it 
does not arise from a well-developed line of reasoning. An ideal (or 
even a "good") set of C0 2 policies will depend on the costs and 
benefits of climate change and C0 2 controls; costs and benefits are 
so poorly understood that no clear line of policy stands out as 
appropriate (see Schelling, Chapter 9). 

In terms of conclusions, the Nordhaus, CEQ, and Perry studies seem 
to be largely consistent in their projections of what emission trajec- 



169 

tories would look like under particular C0 2 -induced constraints. As 
long as fossil fuel growth rates continue at the low level of the past 
few years and concentrations of 400-450 ppm are judged acceptable, there 
is little urgency for significant reductions in CO 2 emissions below 
an uncontrolled path before 1990. Emissions would need to be reduced 
below an uncontrolled path around 2000 if a limit in the vicinity of 
450-500 ppm is desirable. To limit concentrations to 600 ppm (a doub- 
ling from preindustrial levels) would require that serious reductions 
be initiated in the 2010 to 2030 period. These long lead times before 
C0 2 reductions are necessary may be misleading, however. To effect a 
significant reduction of C0 2 emissions in an orderly and efficient 
way probably requires planning and policy measures decades in advance, 
for the infrastructure and capital stock associated with fossil fuels 
cannot quickly be scrapped and replaced without high economic cost. 
Also, it is probably necessary to consider policies with regard to 
climatic change on the basis of possible combined effects of CO 2 and 
other greenhouse gases. 



2.2.5 A Note on the Biosphere 

In the past the biosphere may have been a cumulative source of C0 2 as 
a result of human activities within a factor of 2 as great as burning 
of fossil fuels (Clark et al., 1982? Woodwell, this volume, Chapter 3, 
Section 3.3). However, it appears that in projecting future CO 2 
emissions resulting from human activities the role of the biosphere is 
swamped by the potential contribution of fossil fuel combustion.* 

An estimate for the maximum possible future addition from all 
biospheric sources is 240 Gt of C (Revelle and Munk, 1977). 
Baumgartner (1979) estimates that clearing of all tropical forests 
might contribute about 140 Gt of C. The total carbon content of the 
Amazon forest is estimated at about 120 Gt of C (Sioli, 1973). Chan et 
al. (1980) develop a high deforestation scenario in which total 
additional transfer of carbon from the biosphere to the atmosphere by 
the year 2100 is about 100 Gt of C. The World Climate Programme (1981) 
group of experts adopted a range of 50 to 150 Gt of C for biospheric 
emissions in the 1980-2025 period. Machta (this volume, Chapter 3, 
Section 3.5) estimates that massive oxidation of the biota might 
increase atmospheric CO 2 by 75 ppm by A.D. 2100. 

Projections of future atmospheric CO 2 concentrations embracing 
both burning of fossil fuels and terrestrial sources have all been 
dominated by growth rates in fossil fuel emissions, except in cases 
where fossil fuel emissions are extremely low or in cases like that 
described by Woodwell (Chapter 3, Section 3.6), where all the world's 
forests are entirely destroyed in a few decades. While annual bio- 



*While the role of the biosphere may be marginal in projecting 
future emissions, it is, of course, important in calculating how the 
emissions are distributed among ultimate reservoirs. 



170 

spheric emissions from human activities may average as high as 1 to 3 
Gt of C per year in future decades, fossil fuel emissions are typically 
projected to be an order of magnitude larger. From a different per- 
spective , Schelling (Section 9.2.4) estimates that massive destruction 
or plantation of forests might accelerate or retard the growth of 
atmospheric C0 2 to a particular level by a decade or so during the 
second half of the next century. 

2.2.6 Projections of Non-CC^ Trace Gases 

Changes in atmospheric concentrations of several infrared absorbing 
gases besides CC>2 may result from human activities (see Machta, 
Chapter 4, Section 4.3). These activities include the following: 

(a) Stratospheric flight. Increasing supersonic air traffic may 
lead to changes in the 3 and H20 content of the stratosphere. 

(b) Use of nitrogen fertilizers. Denitrification of fertilizers in 
the soil releases nitrous oxide (N20) to the atmosphere. Less sig- 
nificant increases in NH 3 and HN0 3 may also result. 

(c) Use of chlorofluorocarbons (CFCs) CC1 2 F 2 and CC1 3 F as 
refrigerants and propellants in aerosol spray cans, for example. 

(d) Extraction and burning of fossil fuels. Methane (CH 4 ) may be 
released as a result of mining of coal and extraction of oil and gas. 
CH 4 is also a conversion product of CO, and its presence is thus 
correlated with burning of fossil fuels. 

(e) Agricultural and livestock production. Increasing methane 
emissions may be associated with large livestock herds and expansion 
and intensification of rice production. 

Projections of future emissions of these non-C0 2 trace gases are 
generally at a more primitive stage than are C0 2 projections.* 
Researchers studying biogeochemical cycles and the atmosphere typically 
have used simple assumptions of linear increase or exponential growth 
based on a short segment of recent years (see Flohn, 1980, pp. 22-23). 
Wang et al. (1976), in a widely cited article, assumed that by 2020 
stratospheric H2O, N2p, and CH 4 would all double and that the 
CFCs would increase by a factor of 20. Alternatively, projections may 
be extrapolated from more detailed studies, like the Climatic Impact 
Assessment Program (CIAP, 1975) , which developed scenarios of strato- 
spheric flight. The time horizons of the studies of stratospheric 
flight, agricultural production, industrial use of chemicals, and other 
activities vary, and the macroeconomic assumptions employed vary as 
well. There is a lack of studies of the combined greenhouse effect 
that use assumptions consistently in generating both C0 2 emissions 



*In addition to the references given in the text of this section, 
see Hameed et al. (1980), Lacis et al. (1981), Logan et al. (1978), 
Ramanathan (1980) , and Rowland and Molina (1975) . 



171 

and emissions of other infrared-absorbing trace gases. Given the very 
large inertia and modest rate of technological change in energy systems / 
projection of CC>2 emissions over periods of 50 years and longer has 
large but manageable error bounds. In human activities like use of 
CFCs or stratospheric flight where more rapid technological change is 
occurring/ and where less inertia is imposed by a large and expensive 
capital stock / projections extending many decades are much more 
hazardous. 



2.2.7 Findings 

2.2.7.1 The State of the Art 

2.2.7.1.1 Recent Progress 

Few serious attempts at global long-range energy perspectives were 
undertaken before the 1970s. There has been rapid methodological 
progress in methods of making energy and CO 2 projections over the 
last decade. Much important work is a spinoff from energy analysis 
spurred by the 1973 oil shock. With some exceptions, methods developed 
independent of C(>2 studies in energy modeling/ statistics/ and 
econometrics should be adequate for the task of projecting future 
anthropogenic CO2 emissions/ when brought together with knowledge 
from geology/ engineering/ and other relevant fields. 

2.2.7.1.2 Nature of Modeling Exercises 

Modeling is a way of organizing thinking about a problem/ one that 
should allow improved scrutiny of data/ assumptions/ and relationships. 
There is unlikely to be one "correct" approach to energy modeling for 
CC>2 applications. The systems involved are too complex/ too uncer- 
tain; questions we ask may differ; methodological improvements occur 
frequently. 

Moreover/ there are cultural factors that influence forecasting. It 
is obvious to even a casual observer of the energy scene that there are 
deeply held and diverse views about energy futures/ which are/ after 
all/ views of the character and relationship of man/ nature/ and 
society. Even if all could agree on a single model to use/ we would 
certainly disagree on values for many variables. This is not merely a 
question of uncertainty; it is a question of coexisting contradictory 
certainties/ points about which different groups and individuals hold 
highly assured but also highly different views. 

Historical fashions in forecasting may be equally significant. It 
would be myopic to think that the current set of projections is free of 
today's implicit assumptions or biases. One cannot help but notice the 
tendency in energy forecasting to extrapolate the most recent past/ 
whether one of relatively rapid or slow growth/ far into the future. 
When the price of electricity was going down in the 1950s/ people spoke 
of nuclear electricity becoming too cheap to meter; when the price of 
oil increased in the 1970s/ people spoke of a barrel rising to a price 



172 

of $100 or $200. The tendency of many forecasters to move in parallel 
(so that when one makes an upward or downward turn r all do) is also 
noteworthy. 

How historical trends and trendlines in forecasts should affect both 
choice of method and our interpretation of contemporary forecasts and 
the spread of forecasts remains to be explored further. Probabilistic 
approaches, like that of Nordhaus and Yohe (Section 2.1), are one 
natural response. However, in view of the fickleness of forecasts, it 
is clearly useful to encourage a variety of approaches: large and 
small; formal and informal; stochastic and deterministic. 

A pervasive question in research in CO 2 and energy is how much 
disaggregation is useful for accurate predictions of future C0 2 
emissions. (A similar question arises, indeed, in climate modeling, 
where the question of the optimal level of refinement of spatial grid 
and time steps also occurs.) It is often assumed that more disaggrega- 
tion is better. Careful investigation of the issue shows, however, 
that no general result holds. The potential improvement from disag- 
gregation depends on the purposes of the study, the structure of 
microrelations, and the quality of the microdata. (See Grunfeld and 
Griliches, 1960.) 

There are at least two possible reasons why disaggregation in CO 2 
projections might not produce more reliable estimates. First, disag- 
gregated data may be less reliable than aggregated data. This problem 
can lead to errors in variables and biased statistical estimates of 
microrelations. For example, we might have a good estimate of global 
energy production and, therefore, consumption but not of the distribu- 
tion of global consumption. Second, there may be interdependence 
across regions that would be taken into account in aggregate models but 
not in disaggregated models. An example is the constraint that the 
balance of trade of the world be zero (or the net oil imports of the 
world be zero) . A pasting together of studies of individual countries 
would generally not respect the constraint. In both cases, it is 
possible that aggregate models could provide superior prediction to 
disaggregated models. 

2.2.7.1.3 Assessment of Current Efforts 

The current modeling and knowledge of future C0 2 emissions appears 
marginally adequate today; we have a general idea of likely future 
trends and the range of uncertainty. It may be that further effort 
could increase the accuracy of our forecasts substantially. Given the 
large uncertainty that future energy growth and energy projections are 
contributing to the C0 2 issue, this area may well merit more research 
attention and support than it has received in the past. Future research 
efforts might be designed with four points in mind. 

1. In general, the most detailed and theoretically based projections 
of C0 2 have been a spillover from work in other areas, particularly 
energy studies. This fact suggests that continued support of energy 
modeling efforts will be of importance in further pushing out the 
frontier of knowledge about future C0 2 emissions, as well as the 
interaction between possible C0 2 controls and the economy. 



173 

2. We have identified a serious deficiency in the support of long- 
run economic and energy models in the United States. There is not one 
U.S. long-range global energy or economic model that is being developed 
and constantly maintained , updated with documentation, and usable by a 
wide variety of groups. This shortcoming is in stark contrast to cli- 
mate or carbon cycle models, where several models receive long-term 
support, are periodically updated, and can be used by outside groups. 
Another striking contrast is with short-run economic models, which are 
too plentiful to enumerate. 

3. The bulk of C02 projections have been primitive from a method- 
ological point of view. Work on projecting CC^ emissions has not 
drawn sufficiently on existing work in statistics, econometrics, or 
decision theory. There has been little attention to uncertainties and 
probabilities. Also, considerable confusion of normative and positive 
approaches exists in modeling of C0 2 emissions. 

4. Application of models for analysis of policies where there are, 
for example, feedbacks to the economy from climatic change or CO 2 
control strategies is just beginning. Efforts to evaluate the effec- 
tiveness for C02 control of energy policies of particular nations or 
groups of nations in a globally consistent framework have been lacking. 



2.2.7.2 Likely Future Outcomes 

It is possible to synthesize past work to obtain a likely range of 
future C02 emissions. Before doing so, it is important to reiterate 
the inhomogeneous character of the projections surveyed. 

Some studies, like those of Rotty, Perry and Landsberg, and 
Marchetti, seek to be best guesses or forecasts of future energy demand; 
others, like IIASA, posit scenarios, seek to fill out the descriptions, 
and avoid making claims about probability. Not only do the studies 
vary in intent, they are also of limited comparability in structure. 
The studies differ widely in levels of detail, time horizon, data base, 
and geographical aggregation. While projections of C02 emissions may 
extend to the year 2100, few energy studies offer detailed analysis 
beyond the year 2000, and fewer still offer detail past 2025 or 2030. 
The relative reliance on economic, engineering, and ecological logic 
varies. In addition, the studies are not independent. For example, 
researchers who participated in the IIASA work also participated in IEA 
and Interfutures research; both Lovins and IIASA rely on Keyfitz's 
population projections. 

2.2.7.2.1 Energy Growth 

Figure 2.22 summarizes the energy consumption forecasts to the year 
2030. 

Projections of C0 2 emissions are basically products of projections 
of energy demand and fuel mix. Projections of growth in energy use 
involve, more or less explicitly, assumptions or estimates concerning 
population growth, changes in per capita production of goods and ser- 
vices, and changes in the primary energy input required per unit of 



174 



60 


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YEXFT 

FIGURE 2.22 Past and projected energy consumption. Historical data 
are for primary energy consumption, including noncommercial 
(Nakicenovic, 1979? Schilling and Hildebrandt, 1977; Putnam, 1953). A 
similar figure with a different selection of estimates appears in Clark 
(1982) . 



output. Table 2.21 shows assumptions and estimates that major groups 
have offered. As described earlier, particular assumptions, for 
example, high population growth in Perry and Landsberg (1977) or high 
GDP growth in Nordhaus (1977) , explain much of the resulting projection 
of world energy consumption. 

While the sample of global, long-range energy projections is small 
and uneven and sporadically published, there is evidence of a reduction 
in projected rates of system growth over the past decade (Clark et al., 
in Clark, 1982; Lovins et al., 1982), perhaps spurred by the oil shock 
of 1973. A survey of energy demand projections for the United States 
shows the lower rates of growth expected in a major study from the late 
1970s as opposed to studies in the mid-1970s (see Table 2.22). For 
example, the Rotty projections and even the perenially low projections 
of Lovins decline. 

There are several reasons offered for lowering projected rates of 
growth to levels considerably below the past few decades: a lowering 
of projected rates of population increase, an assumption that economic 
development in developing countries will not imitate the pattern of the 
developed countries, and a reversal in the historical trend toward 



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176 



TABLE 2.22 U.S. Energy Demand Projections by Various Studies* 



Energy 
Study 



Popula- 
tion at 
End of 
Period of Period 



Average 
Annual 

Average Rate of 
Annual Growth , 
Growth, Consump- 
GNP tion 



Projection (millions) (%) 



Total Energy 
in Final 
Year of 
Projection 
(quads) 



Ford Foundation^ 
Historical 
Technical fix 
Zero energy 

growth 

Edison Electric 
Institute 



1975-2000 



1975-2000 



265 
265 
265 



3.02 
2.91 
2.92 



3.4 
1.9 
1.1 



186.7 
124.0 
100.0 



High 


286 


4.2 


3.8 


179 


Moderate 


265 


3.7 


3.2 


155 


Low 


251 


2.3 


1.6 


105 


Exxoi 1977-1990 


ei 


3.6 


2.3 


108 


Bureau of Mines 1974-2000 


264 


3.7 


3.1 


163.4 


EPRlS. 1975-2000 










Baseline^ 


281 


3.4 


3.37 


159 


High electricity^ 


281 


3.4 


4.21 


196.1 


Conservation 


281 


3.4 


2.97 


145.6 


Five times prices 


281 


3.4 


1.98 


114.3 


CONAES 1975-2010 










*2 


279 


2.0 


-0.29 


64 


"2 


279 


2.0 


0.45 


83 


III 2 


279 


2.0 


1.04 


102 


IV 2 


279 


2.0 


1.95 


140 


*3 


279 


3.0 


0.52 


85 


"3 


279 


3.0 


1.38 


115 


III 3 


279 


3.0 


1-.95 


140 


IV 3 


279 


3.0 


2.82 


188 



^Source: National Academy of Sciences (1979). Energy in Transition 

1985-2010 y Final Report of the Committee on Nuclear and Alternative 

Energy Systems (CONAES). National Academy of Sciences, Washington, D.C. 

^Source: Ford Foundation (1974). Energy Policy Project, A Time to 

Choose; America's Energy Future. Ballinger, Cambridge, Mass. 

HSource: Edison Electric Institute (1976) . Economic Growth in the 

Future, Committee on Economic Growth, Pricing and Energy Use. Edison 

Electric Inst., New York. 

^Source: Exxon Company, U.S.A. (1978). "Energy Outlook: 

1978-1990." Available from Public Affairs Department, P.O. Box 2180, 

Houston, Tex. 77001. 

Not specified. 

^Source: U.S. Bureau of Mines (1975). United States Energy Through 

the Year 2000 . rev. ed. U.S. Govt. Printing Office, Washington, D.C. 

^Source : Electric Power Research Institute, (1978) . Demand '77; 

EPRI Annual Energy Forecasts and Consumption Model. EPRI (EA-621-SR) , 

Palo Alto, Calif. 

with restriction on the availability of natural gas. 

iWith no restrictions on the availability of natural gas. 



177 

lower energy costs* These and other factors having to do with either 
the macroeconomic environment or the energy sector can be combined in 
various proportions to form a slower growth in the energy system. 

One additional component in declining energy supply and demand 
projections is probably the reduction in the projected role of nuclear 
energy* Figure 2.23 shows the steady lowering of projections made 
between 1970 and 1978 for 1985 nuclear generating capacity in countries 
of the Organization for Economic Cooperation and Development (OECD) 
(CIA, 1980). Of course, withdrawal of nuclear energy from the supply 
picture may mean substitution of other sources rather than a reduction 
in demand. 

It is interesting to note that none of the major energy-centered 
projections has estimated 4% per year energy demand growth beyond the 
year 2000. This contrasts with the 4.3% figure for growth in CO2 
emissions that prevailed for a time in the carbon cycle and climate 
modeling literature. Most of the studies looking beyond 2000 project 
energy growth between about 2% and slightly above 3% per year. 

The, absolute range of projections spreads strikingly as the time 
horizon is extended. For example/ the range of the more detailed 
projections in the year 2000 is between about 14 TW yr/yr (IIASA low) 
and 21 TW yr/yr (OECD A) , while the range in 2025 is between about 20 
TW yr/yr (IIASA low) and about 40 TW yr/yr (Perry and Landsberg, Rotty) r 
an increase from 7 to 20 TW yr/yr, almost tripling with one generation. 
In fact, in the case of the IIASA low and high scenarios (which are not 
even presented as lower and upper bounds) , the divergence increases 
from about 3 TW yr/yr in 2000 to 13 TW yr/yr in 2030. 

While an individual or group may have a particular preference, the 
review of estimates shows no strong signs of convergence toward a 
single, widely accepted projection or set of assumptions. The analysis 
of Lovins et al. (1981) suggests that the "hard" path of high-energy 
consumption is as far from the "soft" path of low-energy consumption as 
when the energy debate began 10 years ago. While there has been a 
trend downward during the last decade, it appears to be more a parallel 
movement of camps than a convergence (Ausubel, 1982) . 

2.2.7-2.2 Fuel Mix 

While the balance between carbon and noncarbon fuels is obviously 
key to projection of C02 emissions, it is one of the weak points of 
many studies. Only IIASA (1981) , Nordhaus (1979) , and Edmonds and 
Re illy (1983c) make careful attempts to calculate the balance* Other 
studies are notably arbitrary in assigning fuel shares. Of course, the 
fuel mix is likely to have a high degree of intrinsic uncertainty* Over 
a period of 50 years or more, substantial substitution is possible* 
While we know there will still be a need for transportation or home 
heating, how it is accomplished will depend critically on the relative 
prices and availabilities of different fuels. 

Estimates for shares of nonfossil fuel 40 to 50 years hence range 
from 10% (Rotty, 1978) , to 13% (World Energy Conference) , to 25% 
(Marchetti) , to 30% (Edmonds and Reilly) , to almost 35% in the IIASA 
low scenario. The Perry and Landsberg study, where a case of almost 



178 



563 



24 


542 






16 










513 


60 








18 








60 






466 










60 




19 


464 


















18 
















60 
























49 






















400 Thousand Megawatts 






















12 




202 




184 
















41 












175 






































331 


























13 


318 






























13 
















203 












30 
























212 












35 
























167 










259 


































10 




































27 




























141 










214 






























125 








12 


Other 






































18 


Japan 


































107 












































84 


Western Europe 


277 




280 




260 




204 




165 




1BD 




147 




145 




115 




100 


United States 



9-70 8-73 1-74 4-75 12-75 2-76 8-76 1-77 12-77 12-78 
Date of OECD Projection 

FIGURE 2.23 OECD: Past projections of year-end 1985 nuclear 

generating capacity. (Source: CIA, 1980.) 



179 



.2 g s| 

ra - oi C C 



P 8 



T- w 

< I 

52 i 

^ Z 




- p: 



J 8 illlj 

( O) > .2 C JZ > 



SO Q O OT 
CO u) CO .. 





I 
? 

(D 


0) 

I 

M 




J 


r^ 




"C 


^ 






rs 


r> 

fx 


i 


C 




i 


2 


0) 


I 


I 


S 


i 


vt 


i 


5 


t 






=0 


o 
QC 





1 






o 



DC 
LLJ 



D El IS 



I S i 



^ * o 



i i 


till 




CO LO ' 


3 ^D ^^ f^ 

r CO C^ ^^ 






Q) 



CM 0) 



(uoqjeo o sucneBjB) SNOISSIIAI3 NO8dVO 



180 

TABLE 2.23 Some CO 2 Emission Projections Derived from Long-Range 
Energy Projections 



Projections 



2000 2020 2025 2030 2040 2050 



IIASA (1981) 
High scenario 
Low scenario 



10 
7.5 



Niehaus and Williams (1979) 

50 TW high solar/nuclear 10 

30 TW high solar/nuclear 8 

50 TW high fossil 13 

30 TW low fossil 11 



12 
.9 



Rotty (1977) 

Rotty and Marland (1980) 

Perry and Landsberg (1977) 
Coal 
Renewables 



Nordhaus (1977) 10.7 

Marchetti (1980) 8 

Edmonds and Reilly (1983a) 6.9 



26 

14 



27 
13 



18.3 



16 
10 



3 
2 

30 
22 



2 
1 

35 
24 



12.3 



40.1 



14 



10 
26.3 



Nordhaus and Yohe (this 
volume) 50th percentile 



10 



15 



total reliance on coal is contrasted with a strategy where regional 
shortfalls in supply are met by an undefined noncarbon source, is also 
indicative. Here the fossil shares are 96% and 53% r respectively. 

In conjunction with discussion of fuel shares, one other prominent 
feature of long-range energy studies should be mentioned: it is assumed 
or calculated that virtually all easily accessible oil and gas will be 
consumed. While there is contention over rates of depletion, these 
sources are generally posited as too attractive to remain underground. 
(Marchetti, who foresees a phase out of oil before exhaustion of 
resources, and Lovins, who argues for reduced demand and renewable 
substitutes, demur.) Thus, in most studies the estimates of oil and 
gas resources form a minimum expected increase of atmospheric (#2; 
the degree of extraction of coal and shales determines how much further 
the atmospheric buildup of C0 2 will rise. 



2.2.7.2.3 CO 2 Emissions 



181 



Combining estimates of energy and fuel mix leads to projections of 
C0 2 emissions. Figure 2.24 and Table 2.23 show CO 2 projections 
derived from long-range energy projections. Average annual rates of 
increase in C02 emissions to 2030 range from about 1% to 3.5%. 
Estimated annual emissions range from the past studies between about 7 
and 13 Gt of C in the year 2000 and, with a couple of exceptions, 
between about 10 and 30 Gt of C in 2030. 



2.2.8 Conclusion 

Careful analysis of the economy, of energy, and of C0 2 emissions is 
vital. Such efforts are a key to better understanding of how the 
future atmosphere will evolve and what the likely costs and benefits of 
alternative C02 control or adaptation strategies will be. Consider- 
able progress has been made over the last decade in developing more 
reliable and theoretically grounded models. As in other aspects of the 
issue of climate change, in economic and energy modeling a strong 
fundamental research program is a prerequisite for responding in an 
agile way to the concerns of today and images of the next century. 



References 

Allen, E. L., C. Davison, R. Dougher, J. A. Edmonds, and J. Reilly 

(1981). Global energy consumption and production in 2000. 

ORAU/IEA-81-2 (M) . Institute for Energy Analysis, Oak Ridge, Tenn. 
Ausubel, J. H. (1982) . Review of Least-Cost Energy; Solving the C0 ? 

Problem by Lovins et al. (1982). Climatic Change 4; 313-317. 
Baumgartner, A. (1979). Climatic variability and forestry. In 

Proceedings of the World Climate Conference. World Meteorological 

Organization, Geneva. 
Bolin, B. (1979). Climate and global ecology. In Proceedings of the 

World Climate Conference. World Meteorological Organization, Geneva. 
Central Intelligence Agency (CIA) (1980). OECD countries: prospects 

for nuclear power in the 1980s. NFAC/OER/M/IE* Washington, D.C., 

11 July 1980. 
Chan, Y.-H, J. Olson, and W. Emanuel (1980) . Land use and energy 

scenarios affecting the global carbon cycle. Environ. Internat. 

1:189-206. 
Clark, W. C., ed. (1982). Carbon Dioxide Review: 1982. Oxford U. 

Press, New York. 
Clark, W. C., K. H. Cook, G. Marland, A. M. Weinberg, R. M. Rotty, 

P. R. Bell, L. J. Allison, and C. L. Cooper (1982) . The carbon 

dioxide question: a perspective for 1982. In W. C. Clark, ed., 

Carbon Dioxide Review: 1982. Oxford U. Press, -New York. 
Climatic Impact Assessment Program (CIAP) (1975) . The Effects of 

Stratospheric Pollution by Aircraft. U.S. Department of 

Transportation, Washington, D.C. 



182 

Council on Environmental Quality (CEQ) (1981) . Global energy futures 

and the carbon dioxide problem. CEQ, Washington, D.C. 
Edmonds, J. A., and J. M. Reilly (1983a) . Global energy and C0 2 to 

the year 2050. Institute for Energy Analysis, Oak Ridge, Tenn. 

Submitted to the Energy Journal. 
Edmonds, J. A., and J. M. Reilly (1983b) . Global energy production and 

use to the year 2050. Energy 8; 419-432. 

Edmonds, J. A., and J. M. Reilly (1983c) . A long-term global energy- 
economic model of carbon dioxide release from fossil fuel use. 

Energy Econ. 5; 74-88. 
Edmonds, J. A., J. M. Reilly f and R. Dougher (1981). Determinants of 

Global Energy Demand to the Year 2050 (draft) . Oak Ridge Associated 

Universities, Institute for Energy Analysis, Oak Ridge, Tenn. 
Exxon Corporation (1980). World Energy Outlook. Exxon Corp., New 

York, December. 
Flohn, H. (1980) . Possible climatic consequences of a man-made global 

warming. RR-80-30. International Institute for Applied Systems 

Analysis, Laxenburg, Austria. 
Grunfeld, Y., and Z. Griliches (1960). Is aggregation necessarily 

bad? Rev. Econ. Stat. , pp. 1-13, February. 
Hameed, S., R. D. Cess, and J. S. Hogan (1980). Response of the global 

climate to changes in atmospheric chemical composition due to fossil 

fuel burning. J. Geophys. Res. 85:7537. 
Hansen, J., D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and 

G. Russell (1981) . Climatic impact of increasing atmospheric 

C0 2 . Science 213; 957-966. 
Interfutures Project (1979) . Facing the Future. Organization for 

Economic Cooperation and Development (OECD) , Paris. 
International Institute for Applied Systems Analysis (IIASA) (1981) . 

Energy in a Finite World; A Global Systems Analysis. Ballinger, 

Cambridge, Mass. 
JASON (1979) . The long term impact of atmospheric carbon dioxide on 

climate. Technical report JSR-78-07. SRI International , Arlington, 

Va. 
Kahn, H., W. Brown, and L. Martel (1976). The Next 200 Years. William 

Morrow, New York. 
Keeling, C. D. (1973). Industrial production of carbon dioxide from 

fossil fuels and limestone. Tellus 25:174. 
Keeling, C. D., and R. B. Bacastow (1977). Impact of industrial gases 

on climate. In NRC Geophysics Study Committee, Energy and Climate. 

National Academy of Sciences, Washington, D.C. 
Lacis, A., J. Hansen, P. Lee, T. Mitchell, and S. Lebedeff (1981). 

Greenhouse effect of trace gases, 1970-1980. Geophys. Res. Lett. 

i:1035-1038. 
Legasov, V. A., and I. I. Kuz'min (1981). The problem of energy 

production. Priroda(2) . 
Logan, J. A., M. J. Prather, S. C. Wofsy, and M. B. McElroy (1978). 

Atmospheric chemistry: response to human influence. Trans. R. Soc. 

290:187. 
Lovins, A. B. (1980). Economically efficient energy futures. In 

Interactions of Energy and Climate , W. Bach, J. Pankrath, and J. 

Williams, eds. Reidel, Dordrecht, pp. 1-31. 



183 

Levins, A. B., L. H. Lovins, F. Krause, and W. Bach (1982). Least Cost 

Energy; Solving the CO? Problem. Brick House Publishing, 

Cambridge, Mass. 
Marchetti, C. (1980) . On energy systems in historical perspective. 

International Institute for Applied Systems Analysis, Laxenburg, 

Austria. 
Marchetti, C., and N. Nakicenovic (1979). The dynamics of energy 

systems and the logistic substitution model. RR-79-13. 

International Institute for Applied Systems Analysis, Laxenburg, 

Austria. 
Marland, G., and R. Rotty (1979). Atmospheric carbon dioxide: 

implications for world coal use. In Future Coal Supply for the 

World Energy Balance, M. Grenon, ed. Third IIASA Conference on 

Energy Resources, Nov. 28-Dec. 2, 1977. Pergamon, Oxford, pp. 

700-713. 
Nakicenovic, N, (1979). Software Package for the Logistic Substitution 

Model. Report RR-79-12, International Institute for Applied Systems 

Analysis, Laxenburg, Austria. 
Niehaus, F. (1979) . Carbon dioxide as a constraint for global energy 

scenarios. In Man's Impact on Climate, W. Bach, J. Pankrath, and W. 

Kellogg, eds. Elsevier, Amsterdam, pp. 285-297. 
Niehaus, F., and J. Williams (1979). Studies of different energy 

strategies in terms of their effects on the atmospheric CO 2 

concentration . J. Geophys. Res. 84; 3123-3129 . 
Nordhaus, W. D. (1977) . Strategies for the control of carbon dioxide. 

Cowles Foundation Discussion Paper No. 443. Yale U., New Haven, 

Conn. 
Nordhaus, W. D. (1979) . The Efficient Use of Energy Resources. Yale 

U. Press, New Haven, Conn. 
Nordhaus, W. D. (1980). Thinking about carbon dioxide: theoretical 

and empirical aspects of optimal control strategies. Cowles 

Foundation Discussion Paper No. 565. Yale U., New Haven, Conn. 
Nordhaus, W. D. (1982). How fast should we graze the global commons? 

Am. Econ. Rev. 72(2) . 
NRC Geophysics Study Committee (1977) . Energy and Climate. National 

Academy of Sciences, Washington, D.C. 
Perry, A. M. (1982). C02 production scenarios: an assessment of 

alternative futures. In The Carbon Dioxide Review; 1982, W. C. 

Clark, ed. Oxford U. Press, New York. 
Perry, A. M., K. J. Araj, W. Fulkerson, D. J. Rose, M. M. Miller, and 

R. M. Rotty (1982) . Energy supply and demand implications of 

C0 2 . Energy 7:991-1004. 
Perry, H., and H. H. Landsberg (1977). Projected world energy 

consumption. In NRC Geophysics Study Committee, Energy and Climate, 

National Academy of Sciences, Washington, D.C. 

Putnam, P. (1953). Energy in the Future. Van Nostrand, New York. 
Ramanathan, V. (1980) . Climatic effects of anthropogenic trace gases. 

In Interactions of Energy and Climate, W. Bach, J. Pankrath, and J. 

Williams, eds. Reidel, Boston, Mass., pp. 269-280. 



184 

Reilly, J. M. , R. Dougher, and J. A. Edmonds (1981). Determinants of 

Global Energy Supply to the Year 2050 (draft) . Oak Ridge Associated 

Universities, Institute for Energy Analysis, Oak Ridge, Tenn. 
Report of US/USSR Workshop on The Climatic Effects of Increased 

Atmospheric Carbon Dioxide, June 15-20, 1981. Published Leningrad, 

USSR, 1982. Available Division of Atmospheric Sciences, National 

Science Foundation, Washington, D.C. 
Revelle, R. , and W. Munk (1977). The carbon dioxide cycle and the 

biosphere. In NRC Geophysics Study Committee, Energy and Climate, 

National Academy of Sciences, Washington, D.C. 
Rotty, R. (1977) . Present and future production of C0 2 from fossil 

fuels. ORAU/IEA(0) -77-15. Institute for Energy Analysis, Oak 

Ridge, Tenn. 
Rotty, R. (1978) . The atmospheric C0 2 consequences of heavy 

dependence on coal. In Carbon Dioxide, Climate and Society, J. 

Williams, ed. Pergamon, Oxford, pp. 263-273. 
Rotty, R. (1979a). Energy demand and global climate change. In Man's 

Impact on Climate, W. Bach, J. Pankrath, and W. Kellogg, eds. 

Elsevier, Amsterdam, pp. 269-283. 
Rotty, R. (1979b). Growth in global energy demand and contribution of 

alternative supply systems. Energy 4 t 881-890. 
Rotty, R. (1982) . Distribution of and Changes in Industrial Carbon 

Dioxide Production. Institute for Energy Analysis, Oak Ridge, Tenn. 
Rotty, R., and G. Marland (1980). Constraints on fossil fuel use. In 

Interactions of Energy and Climate, W. Bach, J. Pankrath, and J. 

Williams, eds. Reidel, Dordrecht, pp. 191-212. 
Rowland, F. S., and M. J. Molina (1975). Chlorofluoromethanes in the 

environment. Rev. Geophys. Space Ptr y s. 13(1) . 
Schilling, H. D., and R. Hildebrandt (1977). PrimSrenergie-Elektrische 

Energie. Gluckauf, Essen, FRG. 
Siegenthaler, V., and H. Oeschger (1978). Predicting future atmospheric 

carbon dioxide levels. Science 199; 388. 
Sioli, H. (1973) . Recent human activities in the Brazilian Amazon 

region. In Tropical Forest Ecosystems in Africa and South America, 

B. J. Meggers et al., eds. Smithsonian Institution, Washington, D.C. 
Stahl, I.,. and J. Ausubel (1981). Estimating the future input of 

fossil fuel C0 2 into the atmosphere by simulation gaming. In 

Beyond the Energy Crisis Opportunity and Challenge, R. A. Fazzolare 

and C. B. Smith, eds. Pergamon, Oxford. 
Stewart, H. (1981) . Transitional Energy Policy 1980-2030. Pergamon, 

Oxford. 
Voss, A. (1977). AnsStze for Gesamtanalyse des Systems Mensch-Energie- 

Umwelt. Birkhauser, Basel. 
Wang, W. C., Y. L. Yung, A. A. Lacis, T. Mo, and J. E. Hansen (1976). 

Greenhouse effects due to man-made perturbations of trace gases. 

Science 194; 685-690. 
World Climate Programme (1981) . On the assessment of the role of C0 2 

on climate variations and their impact. Report of a WMO/UNEP/ICSU 

meeting of experts in Villach, Austria, November 1980. World 

Meteorological Organization, Geneva. 



185 

World Coal Study (1980). Coal Bridge to the Future. Ballinger, 

Cambridge, Mass. 
World Energy Conference (1978) . World Energy Resources 1985-2020, An 

Appraisal of World Coal Resources and Their Future Availability; 

World Energy Demand (Report to the Conservation Commission) . IPC 

Science and Technology Press, Guildford, U.K. 
World Energy Conference (1980). Survey of Energy Resources 1980. IPC 

Science and Technology Press, Guildford, U.K. 



Past and Future Atmospheric 
3 Concentrations of Carbon Dioxide 



3.1 INTRODUCTION 
Peter G. Brewer 

This chapter on how the carbon content of the atmosphere and other 
reservoirs may change over time has been written in several sections by 
individual authors. In reviewing the material it is clear that all 
controversy in this area has not been resolved. There are three 
principal goals that we seek in our evaluation of the carbon cycle 
among atmosphere r oceans , and biota. First, the climate change that we 
anticipate is based on the atmospheric C0 2 rise. We only have good 
measurements of atmospheric C0 2 from the time of the International 
Geophysical Year in 1958 to the present. We need to know the preindus- 
trial value, the time course of its change in the decades prior to 
1958, and the factors causing this change. Second, we need to know as 
accurately as possible the fluxes of C0 2 among atmosphere, ocean, and 
biota today so as to be able to evaluate contemporary measurements; to 
separate natural, anthropogenic, and climatically modified effects; and 
to identify the role of other greenhouse gases. Third, if our projec- 
tions of the future are to have credibility, we must understand the 
sensitivity of our carbon reservoirs to change and the linkages and 
feedbacks that exist between them. 

We do have reasonable knowledge of the consumption of fossil fuels 
in the early part of this century. If we assume that the airborne CO 2 
fraction has remained constant over this time, then simple backextrapo- 
lation yields an atmospheric CO 2 level of about 290 ppm at the turn 
of the century. Direct measurements of C0 2 in air at that time yield 
equivocal values, having a mean of about 290 ppm and a low value of 
about 270 ppm. Recent measurements of the CO 2 content of air trapped 
in glacial ice indicate a value of about 265 ppm for the middle of the 
last century. If a preindustrial value of 265 ppm is accepted, then 
two things are apparent. First, the discrepancy between the extrapo- 
lated 290 ppm and the observed 265 ppm must be accounted for; the 
difference of 25 ppm added to the atmosphere most likely would come 
from the terrestrial biosphere. There is little evidence for an oceanic 
source, although modest degassing of the ocean would occur on warming. 

Second, the discrepancy is not an insignificant fraction of the 
atmospheric C0 2 rise but accounts for some 30% of the C0 2 signal 
that we see today. The warming due to CO 2 is complex but may be 

186 



187 



approximated by the logarithmic relationship (see Chapter 5) 




where [CO 2 ] is the present C0 2 value, [C0 2 ] is the preindustrial 
valuer and 3.0 represents the mean increase in temperature estimated 
for a doubling of atmospheric C0 2 . The overall warming today then 
could be as high as 1.1C, and the initial C0 2 difference between the 
low and high estimates is 0.4C. In the long run the difference will 
be insignificant; for the present it is uncertain whether we have 
observed a C0 2 -induced warming/ and it is of great interest to know 
the theoretical size and shape of our signal. 

In evaluating the fluxes of C0 2 among our atmospheric, oceanic and 
biotic reservoirs today we note that we have direct measurements only 
of the atmospheric reservoir. Measurements of carbon stocks in the 
global biosphere are complex and are inferred from local measurements, 
patterns of land use, soil changes, and deforestation. In the ocean we 
feel keenly the lack of a high-quality time series of measurements. 
Measurements of the contemporary ocean reveal the large natural C0 2 
cycle and only hint at the anthropogenic signal. Our principal infor- 
mation comes from the observed fractionation of 1 C between air and 
sea from which we calculate oceanic C0 2 uptake. The result is an 
averaged signal, and resolution is poor on time scales less than a 
decade. The principal causes of the annual fluctuations in atmospheric 
CO 2 are the seasonal growth and decay of the terrestrial biota. It 
is as yet difficult to ascribe annual atmospheric fluctuations to 
oceanic changes. One exception to this has been the correlation by 
Bacastow and Keeling of atmospheric C0 2 trends correlated with the 
Southern Oscillation Index. The amplitude of the annual C0 2 
fluctuations revealed at the Mauna Loa observatory shows a tendency to 
increase with time, suggesting increasing terrestrial photosynthetic 
and respiratory activity. 

Ocean-atmosphere carbon models calculate the partioning of fossil 
fuel C0 2 released over the last two decades to be about 40% oceanic 
uptake and 60% atmospheric fraction, with minor net transfers from the 
terrestrial biota. If we express the global carbon balance as 

A a F - s + B (Woodwell, this volume, Section 3.3) , 

where A is the increase in the carbon content of the atmosphere over 
any period, F is the release of carbon to the atmosphere from combustion 
of fossil fuels in the same period, S is the net transfer to the oceans 
in the same period, and B is the absorption or release of carbon by the 
biota in the same period, then the terms S and B are the least well 
determined. The "airborne fraction" is not direc 
either oceanic or biosphere experiments or models. 
changes in the terrestrial biota vary widely from 



.ii 



188 

thinking: If the rate of release of C0 2 from the biota was growing 
at an identical rate to the CC>2 release from fossil fuels/ we would 
find it hard to detect; and if the biotic release was matched by some 
unknown C0 2 sink, the releases could indeed be large. So far we 
cannot unequivocally support either of these scenarios. 

In summary/ the recent estimates of an atmospheric CO 2 concentra- 
tion of about 265 ppm around 1850 lead to a predicted warming greater 
than that yet observed today if we use the upper range of climate model 
results/ and point to a net C02 source from the terrestrial biosphere 
contributing about 25 ppm to the atmospheric levels. This flux from 
the biota most likely occurred in the late nineteenth century and early 
decades of this century. The net release from the biosphere today 
could be about 2 Gt of C per year/ although lesser or negative fluxes 
would not be inconsistent with oceanic and atmospheric models. Finally/ 
we must keep in mind/ as Revelle's analysis (Section 3.5) of methane 
hydrates in continental slope sediments suggests/ that climate change 
may bring about surprising changes in fluxes of carbon. 



3.2 CARBON DIOXIDE AND THE OCEANS 
Peter G. Brewer 



The effect of solution of the gas by the sea water was 
next considered/ because the sea acts as a giant 
regulator of carbon dioxide and holds some sixty times 
as much as the atmosphere. The rate at which the sea 
water could correct an excess of atmospheric carbon 
dioxide depends mainly upon the fresh volume of water 
exposed to the air each year/ because equilibrium with 
the atmospheric gases is only established to a depth of 
about 200 m during such a period. The vertical 
circulation of the oceans is not well understood/ but 
several factors point to an equilibrium time/ in which 
the whole sea volume is exposed to the atmosphere of 
between two and five thousand years. 

Callendar (1938) 



3.2.1 Introduction 

The quotation above is still/ after more than 40 years of progress, as 
succinct a statement of the problem as one could desire. The rising 
atmospheric C0 2 level has been carefully measured since 1958. Cal- 
culations made on the amount of fossil fuel C0 2 released during this 
time, based on good records of oil, coal, and gas combustion, show that 
the observed increase in atmospheric C0 2 is a little more than half 
the amount of fossil fuel C0 2 input. 



189 

The 002 not P resent in the atmosphere must have been transferred 
to some other reservoir, and all investigators who have examined the 
problem over the last four decades have concluded that the ocean is, 
and will remain, the primary sink for fossil fuel 002* The ocean 
holds about 53 times the total atmospheric carbon dioxide content, or 
about 3.7 gigatons of carbon (Gt of 0) as 002* The depth of the 
ocean mixed layer that establishes annual contact with the atmosphere 
is about 75 m, and the mean circulation time for the deep oceans is 
about 500 years (Stuiver et al., 1983). The ocean acts as a "giant 
regulator" not only of 002 but also of climate and thus occupies a 
central role in the debate over the effects of increasing atmospheric 
002 levels on our society. The capacity of the ocean for 002 uptake is 
a function of its chemistry; the rate at which this capacity can be 
brought into play is a function of ocean physics. In addition to these 
direct and present contributions, the deep ocean carbonate sediments 
provide, on a longer time scale, a vast buffer against chemical change. 
The natural vertical gradient of 002 with depth in the oceans is 
driven by the biological flux of particulate matter. 

There have been many recent papers and reviews on these topics (e.g., 
Broecker et al., 1979; Takahashi and Azevedo, 1982), and models designed 
to reproduce their effects (e.g., Oeschger et al., 1975; Killough and 
Emanuel, 1981) . The Scientific Committee on Problems of the Environment 
(SCOPE) reports by Bolin et al. (1979) and Bolin (1981) provide excel- 
lent assessments of the carbon cycle. The scene is one of constant 
research and evaluation; some basic facts, however, hold constant, and 
some uncertainties are widely recognized. These form the basis of this 
review. In attempting to model future atmospheric CO2 levels, the 
largest uncertainty of course surrounds the economic and energy resource 
decisions facing mankind. The C0 2 content of the future atmosphere 
will largely reflect how much C0 2 we choose to put in. The key word 
is choose, for however hard those decisions may be, they represent 
choices distinct from the natural laws that will inevitably be obeyed 
as the CO2 level rises. 



3.2.2 The Cycle of Carbon Dioxide within the Oceans 

Measurements of the carbon dioxide system in seawater plainly reveal 
the natural cycle. If we wish to detect changes in the ocean resulting 
from anthropogenic 002, then a prerequisite is that the natural cycle 
be well understood. This cycle is intimately linked to that of oxygen, 
and the nutrient elements nitrogen and phosphorus, and to ocean 
circulation. 



3.2.3 The Deep Circulation 

The oceans are stably stratified, capped by a warm, less dense surface 
layer and increasing in density with depth. The deep ocean waters have 
a salinity of about 34.9 parts per thousand and a temperature of about 
2C. Most of the deep waters of the world's oceans are formed in 
wintertime in the Norwegian and Greenland Seas and in the Weddell Sea. 



190 

Here winter cooling increases the density of surface waters until the 
stratification of the water column breaks down and f by a poorly under- 
stood process, the deep source regions are renewed. Once formed, the 
bottom waters of the basins exit via various sills and proceed on their 
grand tour. From the Norwegian and Greenland Seas the flow is to the 
south; the residence time of deep water in the Atlantic Ocean has 
recently been estimated as 275 years (Stuiver et al. , 1983) . In the 
southern ocean these deep waters become entrained in the great clock- 
wise circulation of the Antarctic Circumpolar Ocean. Here they branch r 
after a residence time of some 40 years, into either the Pacific or the 
Indian Oceans. The residence time of deep water in the Pacific Ocean 
is about 600 years, and in the Indian Ocean about 335 years. Within 
these ocean basins the deep waters are gradually entrained into the 
shallower flows of the intermediate waters and are eventually returned 
to the surface. The flows are not simply advective, and large-scale 
turbulent processes along and across density surfaces predominate. Reid 
and Lynn (1971) have elegantly shown that the salinity maximum of the 
North Atlantic deep waters can be traced on their trajectory around the 
globe. Stuiver et al. (1983) have followed the decay of radiocarbon 
within these waters and calculated their ages. 



3.2.4 Biological Activity 

Photosynthetic activity by phytoplankton in ocean surface waters fixes 
C02 into organic tissues. The global oceanic annual primary produc- 
tivity is uncertain but is approximately one half that on land 
(Peterson, 1980). The amount, then, is large. Oxygen is produced in 
this process, and ocean surface waters typically show slight (1-2%) 
super saturation with respect to equilibrium with atmospheric 02* 

In contrast to primary productivity on land, where large standing 
stocks of carbon are formed in woody tissues, oceanic production by 
phytoplankton is very rapidly consumed by grazing organisms. Some 90% 
of the organic matter formed is consumed within the euphotic zone. The 
remainder falls through the ocean-water column, as excreted fecal 
pellets and discrete cells, and is subject to oxidative decomposition 
by microbes. Production at the ocean surface is limited largely by the 
availability of the nutrient elements nitrogen and phosphorus. These 
combine with carbon in proportions generally represented by the reaction 



106 C0 2 (g) + 16 N05 + H 2 PO4 + 17 H+ + 122 H 2 - - 
C 106 H 263110 N 16 P + 138 2 (g)- < 

where the subscript g denotes the gaseous state. Although local 
variations in these ratios are found, the mean oceanic signal is 
remarkably constant. The removal of CO 2 from surface waters raises 
the pH; the removal of nitrate and phosphate raises the pH and the 
alkalinity (Brewer and Goldman, 1976) . 

The pattern of oceanic primary productivity is such that large areas 
in the center of the great oceanic gyres are nutrient- impoverished 
oligotrophic regions, where little production occurs. In upwelling 
regions, such as those at the eastern sides of oceanic basins where 



191 

nutrients are brought to the surface, intense biological activity 
occurs. 



3.2.5 Deep Decomposition of Organic Matter 

Below the euphotic zone reaction (1) proceeds in the reverse direction. 
Oxygen is consumed f and carbon dioxide and the nutrient elements are 
released to the dissolved state. The rate of this reverse reaction is 
quite variable. It is generally found to decrease quasi-exponent ially 
with depth, with oxygen consumption rates ranging from about 0.5 ml of 
02/Vyr at shallow depths to less than 0.01 ml of O^L/yr in the abyss 
(Jenkins, 1980). Carbon dioxide is released in proportion to this. The 
result is that the ocean water column is characterized everywhere by an 
oxygen minimum below the euphotic zone, where decomposition is rapid and 
the water column is poorly ventilated. 

Combining these processes with the scheme of the deep circulation 
given earlier, we see a progressive change in the chemistry of the deep 
water during its 500-year deep ocean tour. In traveling from the North 
Atlantic to the Antarctic to the North Pacific, the water becomes sys- 
tematically depleted in oxygen and enriched in CO 2 and the nutrients. 
The GEOSECS series of atlases beautifully reveal these trends as the 
rain of material from above inexorably shifts the oceanic CO 2 chemistry. 



3.2.6 Calcium Carbonate 

The increase in deep ocean C0 2 due to oxidation of organic matter 
lowers the pH of seawater, increasing its corrosiveness to calcium 
carbonate. Surface seawater is supersaturated with respect to calcium 
carbonate, which is secreted by many marine organisms to form their 
shells. These rain down through the ocean to form calcareous sediments. 
Calcium carbonate solubility increases with increasing pressure and 
decreasing temperature, and at some point a horizon occurs below which 
calcium carbonate dissolves. In the North Atlantic Ocean this horizon 
occurs at great depth, approximately 5000 m. Progressing along the 
path of the deep circulation, oxygen and pH are lowered with increasing 
C0 2 levels, and progressively more calcium carbonate is dissolved. 
The dissolution horizon shoals markedly along this trajectory. The 
calcium carbonate thus dissolved from the sediments raises the CO 2 
concentration of the deep water further and increases the alkalinity. 
Of the increase in C0 2 experienced by deep ocean waters, some 70% is 
attributable to decomposition of organic matter and 30% to the dis- 
solution of calcium carbonate. 

Baes (1982) has carefully reviewed this topic. Takahashi et al. 
(1981) have compiled the result of the GEOSECS expedition to illustrate 
the progressive change in these properties. In Figure 3.1 is shown the 
depth distribution of C0 2 in seawater for the various ocean basins 
from the North Atlantic to the North Pacific. In Figure 3.2, the 
equivalent change in alkalinity is shown. 

With this background in mind we now examine some details of the 
chemistry of seawater. 



192 



I r 



O 
O 




dP 

CO * 

r* H 

C 2 
H * 

CO * 

W C 



0) -H 

01 C ^ XI 

H -P O 

C H 3 CO 

H flj O* 

(0 01 H 

o c * * 

H flj H 

-P (U CO H 

as g td 



c 

0) 

o 



CO 



4J At 

3 EH 

Se 



C O O 



U ro O 

(0 <U U4 

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H O -H 

H H -H W 

(d fl3 4J -H 

4J g O O 

q u y nj 



0) 

50) 
U 
(0 

iw 
o w 



M 




5C H 
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O 30 

-H <u O (d 

cn* 



U CO P 

P W 

01 C ;j w 

H -H O 



C H * 



^- ro 



^8 



Ul M H 



Hld3Q 



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C9 i"i * )*4 



193 



O 

in 



OJ 




CO 



<tf 



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Li nrt 

"T ^"* 

o O\ 

> rH 

H r 

rH 

s fl 

-p 4J 
<d CD 

.H 

<D x: 

CO CO 



I- 

4J 0) 
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0) D 



.C IM 
Ou CQ 



2 



-P 
CQ O 

= 



Hld3Q 



CO (TJ 

sal 



194 
3.2.7 The Chemistry of C0 in Seawater 

The reaction of gaseous CO 2 with water produces hydrated C0 2 and 
carbonic acid as in 

C0 2 g + H 2 - + H 2 C0 3 . ( 2 ) 

The carbonic acid may dissociate by losing hydrogen ions as in 
H 2 C0 3 - ^ H+ + HCO^ (3) 

and 

HC05 - *- H+ + COf. ( 4 ) 

with the relative proportions of these species at any time being set by 
the pH of the system. This representation is somewhat crude , and a 
great many minor species also contribute to the acid-base balance of 
seawater, in particular the boric acid equilibrium 



B(OH) 3 + H 2 



(5) 



The addition of C0 2 to seawater changes its chemistry in 
accordance with established laws as ocean and atmosphere strive to 
attain equilibrium. 

Any large body of water will tend toward equilibrium with atmo- 
spheric CO 2 ? the unique feature of the oceans, in addition to their 
enormous size, lies in their alkalinity. The alkalinity of seawater 
arises from the dissolution of basic minerals in seawater, principally 
calcium carbonate. Alkalinity is operationally defined as the amount 
of acid required to titrate 1 kg of seawater to a constant pH value 
corresponding to conversion of bicarbonate and carbonate ions to car- 
bonic acid. In practice very high precision in measurement is required 
for useful work and a precise equivalence of reactions (2) through (5) 
above is sought, not simply a pH value, so that the acid present exactly 
balances the bases as in 

[H + l = [HC03] + 2 [COf] + [B(OH)4] + [OR-]. ( 6 ) 

The alkalinity of the present-day oceans is reasonably well known. It 
has been measured as a basic component of the GEOSECS (Takahashi et 
al., 1980a) and TTO expeditions (PCODF, 1981). The alkalinity of ocean 
surface waters is quite well correlated with salinity; at a salinity of 
35/oo it is approximately 2300 equivalents/kg. The review by 
Skirrow (1975) provides a comprehensive and scholarly account of ocean 
C0 2 chemistry, and the paper by Bradshaw et al. (1981) illuminates 
the complexity of ocean C0 2 system measurement. 

The principal effect of adding C0 2 to ocean surface water is to 
consume carbonate ion: 



C02 + CO~ + H 2 - >-2HC03. (7) 



195 

The reaction does not proceed entirely to the right, and considerable 
resistance to change occurs. This resistance is accurately reflected 
in the thermodynamics of the C02 system. The buffer factor/ or 
Revelle factor as it is widely known, appropriate for this reaction may 
be presented by 

(dpC0 2 /pC0 2 ) TA,T,S 

R, (8) 



(dTC0 2 /TC0 2 ) TA,T,S 

where TC0 2 is the total concentration of carbon dioxide in all its 
forms, pC0 2 is the partial pressure of carbon dioxide gas, TA is the 
total alkalinity, T is the temperature, and S is the salinity. 
Sundquist et al. (1979) have pointed out that this property is quite 
well known. It varies with temperature and has a numerical value of 
about 10. In essence a 10% change in pC0 2 produces only a 1% change 
in C0 2 . 

Takahashi et al. (1980b) have described the change in this factor 
that will inexorably occur as ocean C0 2 levels rise. Figure 3.3 
shows the change as function of C0 2 for alkalinities of 2.2 and 2.4 
milliequivalents/kg, taken from their paper. As the CO 2 content of 
the atmosphere and therefore the surface ocean increases, we move to 
the right on this figure encountering sharply rising values of R. The 
resistance to change increases, the ocean absorbs proportionately less 
C0 2 , and the airborne fraction rises. This is a complex system, 
sensitive to the alkalinity/total CO 2 ratio, and hence pH. The sharp 
maximum that occurs in Figure 3.1 is readily understandable in terms of 
carbonate chemistry equilibria (Takahashi et al., 1980b) and occurs 
when the concentration of 003 becomes equal to that of H 2 CO3; 
thereafter a decrease in R will take place with R asymptotically 
approaching 1. 

How accurately is the curve in Figure 3.3 defined, what processes 
are likely to alter it, and how will we know if the ocean does indeed 
proceed along the thermodynamic course that we have charted? The curve 
is a theoretical construct, based on sound principles of solution 
chemistry* We would like to have a series of field observations of the 
varying concentration of CO 2 in the ocean with time so as to follow 
these changes; however, there are no adequate measurements for this 
purpose. In practice, the buffer factor is not a measured variable but 
a calculated property. 

The accuracy of these calculations depends on our knowledge of the 
solubility of CO 2 gas in seawater and on the thermodynamic constants 
describing the dissociation of carbonic and boric acids in seawater. 
Although these have long been investigated, it is only relatively 
recently that results of sufficient accuracy have been obtained. 

The solubility of CO 2 gas has been determined by Murray and Riley 
(1971) and by Weiss (1974). The results of these experiments are in 
excellent agreement and have been fitted by Weiss (1974) to the equation 

In a 1 = -60.2409 + 93.4517 (100/T) + 23.3585 In (T/100) 

+ [0.023517 - 0.023656 (T/100) + 0.0047036 (T/100) 2 ]S, (9) 



196 



844/iatm 



1 91 7/iatm 



18 




0C 



SCO 2 ID' M/kg 

FIGURE 3.3 Variation of the buffer factor, or Revelle factor, (R) of 
seawater with changing total C0 2 . The calculation is for seawater of 
35% salinity and a total boron content of 0.41 mM/kg. Curves for 
waters of two different alkalinities are shown. Increasing CO 2 
levels raise the buffer factor and diminish the oceans tendency to 
absorb C0 2 . (From Takahashi et al. f 1980b.) 



where a ' is the solubility in mol/kg of seawater /a tin, T is the 
absolute temperature, and S is the salinity. This solubility equation 
has been used in virtually all recent models of ocean 002 uptake. 
The solubility of C0 2 is much greater than that of 2 or N 2 ; the 
relative proportions of N 2 :0 2 :C0 2 in the atmosphere are about 
2400:630:1, whereas in seawater the corresponding ratios are 28:19:1 
depending on the salinity and temperature (Skirrow, 1975) . There is 
little uncertainty in our knowledge of CO 2 solubility. 

The dissociation constants (K-j^ and K 2 ) of carbonic and boric 
acids in seawater have had a rich investigative history, and a complex 
literature attests to this. The dissociation constants are formally 
defined as 



197 

H + [HCC>3] 

- (10) 

[H 2 C0 3 ] 



and 



K 2 = - f (11) 

[HC03] 

where the square brackets denote the concentration of the species in 
seawater. These are apparent, not true, thermodynamic constants 
combining the activity of the hydrogen in with the concentrations of 
the C02 species. Much of the difficulty has surrounded the 
definition of the pH and ionic medium scales used by the various 
experimentalists who have determined these constants. Also, the lack 
of any convention for fitting the data obtained to mathematical 
functions has resulted in an arcane set of equations. 

Millero (1979) has reviewed this situation. He finds that the data 
of Lyman (1956) , Hansson (1973) , and Mehrbach et al. (1973) all yield 
very similar results when the apparent constants (Kj^) for the 
equilibria at various salinities (S) are fitted to equations of the form 

In Ki = In K iw + AiSi/2 + Bi S f (12) 

where Ki is the constant for pure water, and A A and E^ are temperature- 
dependent adjustable parameters. The discrepancies in calculated values 
of [HCO^] and [0:03] for water of fixed alkalinity and total CO 2 are 
about +10 y mo I/kg among the various constants. These differences 
have almost no effect on our ability to model ocean CO 2 uptake but 
are a considerable irritant to researchers attempting to make and 
verify accurate C0 2 measurements under often trying field conditions. 

In one area there is a degree of uncertainty: the dissolved organic 
matter in natural seawater is not represented in any way in these for- 
mulations. Typical dissolved organic matter concentrations in ocean 
water are 1 mg of C/kg (83 y mo I/kg) . The acid-base characteristics 
of this material, and thus its contribution to the alkalinity, are 
poorly known. Huizenga and Kester (1979) report about 11 pmol of 
sites per mg of C on this material with a dissociation constant of 
about 10 3 - 5 . The effect then is not likely to be large. 

The preceding paragraphs show that for a seawater of constant 
alkalinity and chemical composition we can calculate quite well the 
effects of adding CC>2. However, changing the alkalinity of seawater 
will have a marked effect. Adding CO 2 gas to seawater [Equation (7)] 
does not change the alkalinity since charge balance is not altered? the 
dissolution or precipitation of CaCC^ [Equation (6) ] does. 

The principal forms of CaCO 3 in the ocean are the polymorphs calcite 
and aragonite, and these are secreted by calcareous organisms to form 
their shells. Surface seawater is greatly supersaturated with respect 
to both calcite and aragonite, spontaneous precipitation being ki- 
netically inhibited. The various chapters in the volume edited by 



198 

Andersen and Malahoff (1977) testify to the complexity surrounding 
CaC0 3 formation and dissolution in the oceans. The solubility of 
CaC0 3 in seawater increases with increasing pressure, decreasing 
temperature, and decreasing pH; thus, the deep oceans are under sat- 
urated with respect to CaC0 3 , and dissolution occurs. 

As we add CO 2 to the surface ocean we decrease the pH and increase 
the tendency for CaC0 3 dissolution. If this dissolution occurs, then 
both the alkalinity and the total C0 2 increase; although this process 
generates an increase in total C0 2 , the net effect of the alkalinity 
increase would be to enhance the ocean's capacity for C0 2 uptake by 
maintaining constant the factor R (Figure 3.3) and providing C0 3 
ions. 

Model calculations tend to assume a constant alkalinity scenario, 
and there is no evidence that the alkalinity of the ocean has increased 
in recent times. A skeptic could however point out that there is 
precious little evidence that it has not. Ambiguities in definition of 
alkalinity [e.g., the inclusion of minor species such as HPOf" and 
SiO(OH) 3 ], imprecision in measurement, and lack of a historical time 
series leave us with a poor temporal record of the alkalinity and total 
CC>2 content of the ocean. 

Several things make this problem complex. First, the concept of the 
solubility of pure CaCO 3 in seawater is moot; Morse et al. (1980) 
show that the surface undergoing dissolution rapidly becomes transformed 
into a Mg-Ca-CO 3 interfacial layer with complex kinetic and solubility 
controls. Second, biogenically produced magnesian calcites (such as in 
some algae and the spines of sea urchins) , containing 15 mol % magnesium 
or more, commonly occur in ocean surface waters. The stability of this 
material is poorly understood, and its dissolution would change alka- 
linity. Garrels and Mackenzie (1981) recently reviewed the susceptibil- 
ity of magnesian calcite phases to C(>2-induced dissolution. Their 
conclusion was that insufficient magnesian calcites existed globally to 
have major impact, if dissolved, on ocean C0 2 uptake. Dissolution of 
this material, however, would certainly be noticed on a local scale. 

Finally, since the surface ocean is so strongly supersaturated with 
respect to calcite and aragonite and is likely to remain so, it is 
widely assumed that no dissolution of these minerals takes place there. 
Aller (1982) has pointed out that this is not so. Calcareous shells in 
near shore muds are exposed to interstitial waters rich in respiratory 
C0 2 and low in pH. The shells are dissolved quite rapidly, resulting 
in a diffusive flux of Ca 2 + and CO| ions to the overlying waters. 
As we change the C0 2 content of surface waters, we change the upper 
boundary condition controlling this flux. The effects of this process 
are still being explored. 

It is quite possible to incorporate the effects of a postulated 
increase in ocean alkalinity into an atmosphere-ocean C0 2 model, such 
as has been done by Bacastow and Keeling (1979) , although the accuracy 
of the conclusions is uncertain. In this calculation, the dissolution 
of deep calcium carbonate was found to have little immediate effect on 
rising atmospheric C0 2 levels, since the affected seawater would be 
sequestered in the deep ocean. As this water is brought into contact 
with the atmosphere, it will slowly draw down the atmospheric C0 2 



199 

levels some hundreds of years in the future. If we were to dissolve an 
average depth of 3 cm of pure calcium carbonate from the ocean floor , 
then in 1500 years the atmospheric C0 2 level would be some 30% lower 
than with no change in alkalinity. If shallow-water calcium carbonate 
dissolves, the effects are more dramatic. If we were to dissolve an 
average depth of 40 cm of pure calcium carbonate from shallow-water 
sediments, then the peak atmospheric CO2 level would only be some 60% 
of that with no dissolution occurring, and the drawdown in the future 
would be more rapid. 

This, as emphasized by Bacastow and Keeling (1979), is of course 
unrealistic. There are kinetic limits and controls on carbonate 
dissolution extrinsic to the model considered here (e.g., Sayles, 1981; 
Emerson and Bender, 1981), and the disappearance of such massive amounts 
of carbonate shells and corals and sediments from our shores and shallow 
seas would present a crisis for man arousing far greater concern than 
any incremental effect on C0 2 levels. 

In the future it appears inevitable that fossil fuel CO 2 -induced 
dissolution of calcium carbonate will take place in the ocean. The 
most sensitive site appears to be in the deep North Atlantic Ocean 
where waters enriched in industrial CO 2 begin their deep ocean tour 
(Broecker and Takahashi, 1977) and conceptually, since these waters 
have demonstrably "seen" fossil fuel CO 2 , some dissolution has likely 
already occurred. However, there is no time series of measurements in 
the oceans adequate to confirm or deny these statements, and some 
thought must be given to this if progress is to occur. 



3.2.8 Measurements of Ocean CQ ? 

Ocean surface water today contains about 2000 ymol/kg of CO 2 . The 
amount varies with temperature, location, and season. We do not know 
the concentration in the past. The atmospheric C0 2 content in the 
last century appears to have been about 270 ppm, although uncertainty 
exists. Assuming the maintenance of overall equilibrium, we can 
calculate that modern-day surface ocean waters must contain about 35 
pmol/kg more C0 2 than in the past, an approximate 1.8% increase. 
Although this appears to be so, there is no time series of ocean CO 2 
measurements adequate to support this claim. The ocean surface pH is 
similarly variable in the range 8.0-8.3; we can calculate that ocean 
surface pH has been reduced by about 0.06 pH units. The carbonate ion 
content of surface seawater depends on the complex equilibria estab- 
lished between the species H 2 C03, HCO^ and CO|; it is 
typically 15% or so of the total C0 2 concentration. We calculate 
that ocean surface water today contains about 10% less carbonate ion 
than in the past. 

How well can we measure these changes? The most problematic and 
least satisfactory measurement is pH; a measurement precision of +0.01 
pH unit is possible, but the long-term change would be hard to detect 
over seasonal fluctuations. 

Alkalinity can be measured with a precision and accuracy of about 3 
y equivalents/kg. The total CO 2 content can be determined to +4 ymol/kg 



200 

by potentiometric titration (Bradshaw et al, 1981); recently 
Keeling (1983) has presented ocean CO 2 data based on gas extraction 
and manometric measurement accurate to +0.5 ymol/kg. The current 
rate of increase of the total C0 2 concentration in ocean surface 
waters is calculated to be approximately 1 ymol/kg/yr. Plainly such 
an increase would be observable from time series measurements made 
today within the space of a few years. 

The most sensitive measurement is pC0 2/ which may be determined to 
within a few tenths of a part per million. By pC0 2 we mean the 
pressure of C0 2 gas that would be found in a small volume of air that 
had been allowed to reach gaseous equilibrium with a large volume of 
seawater. Surface seawater strives to maintain a pC0 2 globally in 
equilibrium with the atmosphere, lagging behind by some small value due 
to the finite time required for gas exchange to take place. There is a 
large natural variability in ocean surface pC0 2 (Keeling 1968; Miyake 
et al., 1974; Takahashi, 1979) as shown in Figure 3.4. 

The data shown in Figure 3.4 are in fact the deviations of ocean 
surface pCO 2 from equilibrium with the atmosphere. It is not a 
synoptic data set but an ensemble of results from many expeditions, 
quasi-normalized in time and containing considerable seasonal noise. 
The essential features are strongly negative values at high latitudes, 
where rapid cooling and biological activity have markedly lowered 
pC0 2 , and high values at the equator, where upwelling of CO 2 -rich 
water and warming have raised pCO 2 . Gaseous exchange of CO 2 is 
sufficiently slow that equilibrium with the atmosphere is not achieved 
locally, and these patterns persist. Negative values imply invasion of 
CO 2 from the atmosphere to the ocean; positive values indicate 
evasion of C0 2 from the ocean to the air. 

This distribution results in a net flux of C0 2 between the equa- 
torial and polar oceans (Bolin and Keeling, 1963). Pearman et al. 
(1983) have recently calculated that the net release of C0 2 by the 
equatorial oceans is currently about 1.3 Gt of C/yr; the net uptake by 
the high-latitude oceans is about 4.4 Gt of C/yr. The presence of 
fossil fuel CO 2 has enhanced the polar uptake and suppressed the 
equatorial source. 

There is now evidence for a systematic change in ocean surface pC0 2 
with time. Takahashi et al. (1983) have compiled their measurements of 
pC0 2 in Sargasso Sea surface waters from the IGY (1957) , GEOSECS 
(1972), and TTO (1981) expeditions. The results are shown in Figure 
3.5, together with the atmospheric trend. It is clear that ocean 
pC0 2 is rising; however, the interpretation of this signal requires 
caution. The problem is not with the precision of measurement but with 
compensating for the natural noise due to fluctuating ocean surface 
conditions. The atmospheric and oceanic lines are not parallel in 
Figure 3.5, and some explanation of this is needed. The Sargasso Sea 
exhibits low surface pCO 2 with respect to the atmosphere (Figure 
3.4). This arises from the marked negative heat flux observed there 
(Bunker and Worthington, 1976) , so that surface seawater is cooled 
during its residence time in the Sargasso Sea. The slope of the 
oceanic line in Figure 3.5 thus represents not only the anthropogenic 



201 




HCO 

O CQ g 

M CD O 

IM 3 M 

H 14-1 
0) (0 

J-l > <N 

P CD 8 



CO 



CQ 



CO CD 

O CD 

CD O CD 

CQ V4 4J 

CQ 9 CO 

CD O O 

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CD C -H 

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CD CD 

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CQ CO CD 



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T - I II II | I I I || | I f 



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ATMOSPHERIC C0 2 



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05 

en 



o 

CD 
CD 



YEAR 



FIGURE 3.5 The change in the mean pC(>2 of surface Sargasso Sea water 
from the IGY expedition (1957) to the TTO-North Atlantic expedition 
(1981). (From Takahashi et al. , 1983.) 



CO 2 trend but climatic variations as well, suggesting perhaps a 
modest warming during this period. 

The increase in ocean surface pCO 2 is readily measurable from 
time-series measurements made today. There appears to be little reason 
to maintain in the future the lack of knowledge of trends in ocean 
surface CO 2 properties that has characterized the past. 



3.2.9 Models of Ocean CO ? Uptake 

Mathematical models describing the transfer of C0 2 among atmosphere, 
biosphere, and ocean are an essential tool to scientists working in 
this field. The features of ocean CO 2 chemistry reviewed above have 
long been recognized and are adequately represented in recent models 
(Oeschger et al., 1975; Bacastow and Keeling, 1979). Future refinement 
is probable; however, the essential chemical concepts are in place. 



203 



How do we model ocean C0 2 uptake, what data are available for 
model testing, and how do we know if the models are correct? 

The physical basis for the modeling of ocean C0 2 uptake is 
reasonably well established. The first criterion is that the gas 
exchange rate be known. Broecker and Peng (1974) have examined the 
problem of determining gas exchange rates, and Broecker et al. (1979) 
have evaluated the effects of uncertainty in this property on cal- 
culations of ocean C0 2 uptake. Most field data have been obtained by 
the radon technique by which the deficiency of naturally produced 
radon-222, due to gas exchange at the sea surface, is measured. A 
simple stagnant film model is used to calculate the gas exchange 
parameter and to relate the observations to C02 exchange. The 
two-way C0 2 exchange rate obtained in this way is 16 mol/m 2 /yr. A 
much larger-scale estimate is available from the natural balance of 
14 C by radioactive decay in the interior of the ocean must be balanced 
by a fresh influx of 14 C0 2 . The result from this calculation 
yields a rate of 19 + 6 mol/m 2 /yr. C0 2 exchange in a wind-wave 
tunnel has been examined by Broecker et al. (1979), yielding results in 
substantial agreement with the above. 

The calculations of gas exchange rates appear to rest on sound 
principles. The characteristic exchange time for CO 2 is about 10 
times longer than for the nonreactive gases (N 2 and 2 / for example) 
and is about 1 year (Broecker and Peng, 1974). The time scale for 
L4 CO 2 exchange is about 10 years, owing to the time required for 
complete isotopic equilibration with the carbon pool. Gas exchange for 
CC>2r though quite slow, does not appear to be the rate-limiting step 
in models of ocean C0 2 uptake. 

The rate of vertical mixing in the sea is widely viewed as the 
critical parameter. The annual cycle over most of the ocean is such 
that in the summer the sea becomes capped by a warm surface layer, which 
undergoes continuous gaseous exchange with the atmosphere. Wintertime 
cooling and late winter storms increase the density of this upper layer 
so that it finally becomes unstable and undergoes turbulent exchange 
with waters of equivalent density below. This scheme of wintertime 
surface turnover, followed by lateral penetration along surfaces of 
constant density, is what is parameterized in various ways in ocean 
models. Most water mass formation takes place in a location and at a 
time when we cannot observe it. Some integrated measure of the effect 
is required, and this has been provided by the tracer approach. 

The principal tracers used have been 14 C and 3 H; the transcendental 
virtue of 14 C is that it identically mimics C0 2 , taking part in both 
the gaseous and biological exchange cycles. Revelle and Suess (1957) 
and Craig (1957) used 14 C box models to evaluate carbon dioxide 
exchange between atmosphere and ocean and within the ocean. As these 
models grew in complexity (Keeling and Bolin, 1967, 1968) , the neces- 
sity for multiple tracers became apparent, as did the difficulty ot 
assigning realistic transfer rates between boxes based on objective 
experimental evidence. The injection into the atmos P h ^" ^^ SS1 
amounts of radionuclides in the nuclear bomb tests of 1958-1962 
to the signing of the nuclear test-ban treaty P rovided u 3U ^ 
suite, and a decade later ocean scientists organized the GEOSECS 



Iff 



204 



WESTERN NORTH ATLANTIC 

0* 10* 20* 

J 



GEOSECS TRITIUM 



CTU81N) 

80'N 



1000 



2000 



I 
a. 

UJ 

o 



3000 



4000 



5000- 



6000 




FIGURE 3.6 Penetration of tritium into the North Atlantic Ocean in 
1972 along the GEOSECS cruise track. (From Ostlund et al., 1974.) 



experiment to observe the oceanic distribution of these species. The 
tritium and radiocarbon results from this program (Ostlund et al., 
1974) provided a snapshot of ocean tracer penetration on a 10-year time 
scale; Figure 3.6 shows tritium penetration into the Atlantic Ocean in 
1972. 

The results shown in Figure 3.6 represent a vertical slice through 
the western basin of the North Atlantic. They plainly reveal the 
formation of new deep waters; however , such a representation 
understates the great horizontal extent of the oceans. C0 2 is taken 
up by surface seawater globally, and between 50-70% of the ocean's 
anthropogenic C0 2 burden is stored in the surface and thermocline 
waters of the great oceanic gyres (Stuiver, 1978; Siegenthaler, 1983). 

Although the penetration of tritium (Figure 3.6) provides a vivid 
pictorial representation of the extent of the ocean labeled by an 
invading tracer in the 10-year period 1962-1972, the conversion of this 
signal to that of C0 2 is complex. First, tritium was injected as a 
pulse early in the decade, and largely in the northern hemisphere. 
Second, the mean age of the fossil fuel C0 2 increase is about 28 



205 



WESTERN NORTH ATLANTIC 
10 



TTO TRITIUM (TU81N) 
6.0 7.0 8pN 




6000 



FIGURE 3.7 The re-occupation of the GEOSECS stations in 1981 on the 
TTO Expedition. Tritium was principally injected into the atmosphere 
of the northern hemisphere by nuclear bomb tests in 1962. The figure 
illustrates the labeling of the ocean by the invasion of a passive 
tracer in a 19-year period. The mean age of the fossil fuel CO 2 
signal is about 28 years. (From Ostlund, 1983.) 



years (Broecker et al., 1979). What fraction of the ocean will be 
labeled on the 28-year time scale appropriate for C0 2 ? Fortunately , 
the experiment shown in Figure 3.6 has been repeated. In Figure 3.7, 
results are shown for a reoccupation of this section in 1981, 19 years 
after the principal tritium pulse (Ostlund, 1983) . The progression of 
the deep-water front is plainly to be seen. 

These data provide a critical test for models of ocean CO 2 uptake, 
for unless the models can match these observations, they are unlikely 
to be correct. It should be noted, however, that the converse is not 
necessarily true. A model that simply matches data at one point in 
time and contains unrealistic physical principles or dynamic 
characteristics cannot be said to be "true" or "correct" in any 
satisfactory sense. 



F'T ' 



206 

Oeschger et al. (1975) gave major impetus to this field with their 
successful implementation of a box diffusion model. The model con- 
sisted of an atmosphere, a biosphere, an ocean mixed layer, and a 
diffusive deep ocean. Their detailed evaluation of the transfer 
equations and careful selection of numerical properties (mixed layer 
depth 75 m, eddy-diffusion coefficient =1.3 cm 2 /sec) yielded a 
good match to the atmospheric signal. The model successfully coped 
with the different dynamic responses required for penetration of bomb 
14 C and excess fossil fuel C0 2 , and they concluded that the model 
would be valid for predictions of the atmospheric C0 2 response to the 
various possible future C0 2 input time functions. 

Killough and Emanuel (1981) recently compared several models of 
ocean CC>2 uptake, the models differing principally in the size and 
number of reservoirs and their sequence of interconnection so that the 
dynamic response characteristics varied. Bach of the models was 
calibrated from observations of natural 14 C activity and tuned to 
match the observed response to the penetration of bomb 14 C. Their 
evaluation gave results in substantial agreement with those of Oeschger 
et al. (1975), Stuiver (1978), and Broecker et al. (1979). 

At this point it is clear that virtually all successful models of 
ocean CC>2 uptake have relied on the tracer approach, particularly 
14 C and tritium. The models are tuned to simulate the natural 
radiocarbon distribution and their dynamic response tested against the 
bomb transient. 

The tracer, rather than direct, approach has been necessary for 
several reasons. First, there is no time series of ocean C0 2 
measurements of high accuracy that would match the Mauna Loa and other 
atmospheric records. Early oceanic CO 2 measurements quite simply 
lack accuracy and precision and rest on an unsatisfactory thermodynamic 
basis. Second, models depending purely on physical principles and 
measurements (T,S) are frequently undetermined. Without the constraints 
supplied by independent tracers of differing source functions, satisfac- 
tory solutions are not easily achieved. Finally, measurements of ocean 
CO 2 made today are inadequate on their own to solve the fossil fuel 
C0 2 problem. 

The models described above, which currently serve to calculate ocean 
C0 2 uptake, while highly ingenious, are nonetheless viewed with 
considerable skepticism by physical oceanographers. The concern is not 
that the overall estimate of ocean C0 2 uptake is substantially in 
error but that the parameterization of all of ocean physics into a 
single vertical eddy-diffusion coefficient is unacceptable. Garrett 
(1979) has carefully reviewed the evidence for vertical diffusion in 
the ocean and finds no physical basis for a vertical diffusion coef- 
ficient of >_! cm 2 sec"" 1 as required by Oeschger et al. (1975), 
and indeed by all one-dimensional vertical models. A value of one 
tenth that number appears realistic. Jenkins (1980) has followed the 
time history of the penetration of tritium, and its stable, gaseous 
daughter product helium-3, into the Sargasso Sea. He shows unequivo- 
cally that a one-dimensional model with high vertical diffusivity 
cannot explain the results and that a scheme of wintertime surface- 
water turnover followed by lateral penetration along density surfaces 
is required. 



207 

There are of course many models of ocean circulation that give a 
more realistic portrayal of ocean physics (Holland , 1971, 1978) , and 
similar models are now being applied to study the time-dependent 
penetration of tracers into the interior of the ocean (Sarmiento, 
1982} . The application of these models to the CC>2 problem will be 
complex but seems to be possible. Rapid progress is to be expected in 
this field. However, the principal criticism of ocean CO2 uptake 
models has come not from their representation of ocean physics but from 
their failure to include some components of the natural, or perturbed, 
CO 2 cycle. 

The basic assumption for models such as those of Oeschger et al. 
(1975) or Killough and Emanuel (1981) is that the natural cycle of 
C0 2 within the ocean has been unchanged by the activities of 
man i.e., primary production remains at past levels, and the vertical 
flux of particulate carbon has not been altered. The initial ocean 
C(>2 profile thus appears in the models as a constant value. The true 
ocean CO 2 cycle is enormously complex, so that the simplification 
introduced by this procedure is most attractive. The recent report by 
Bolin et al. (1982) , who have incorporated realistic profiles of 
oceanic phosphate, oxygen, CC>2, alkalinity, and 14 C in a highly 
developed 12-box ocean model, illustrates this point well. The carbon 
dioxide concentration in the North Atlantic has been measured in 
parallel with the tritium section in Figure 3.7 (Brewer, 1983). The 
results (Figure 3.8) show the large natural variability of the oceanic 
C0 2 system. Hidden within these data lies the increase in CO 2 
caused by the activities of man. 

Kempe (1982) has reviewed the long-term trends in river fluxes to 
the ocean of carbon and nutrients in great detail and has shown that 
fertilization, land use, and industrial activities have altered many of 
these fluxes markedly. Walsh (1981) has suggested that overfishing off 
Peru and other perturbations (Walsh et al., 1981) have changed the 
storage of carbon on continental shelves. It is clear that the facets 
of carbon research are so many and varied that to attempt to satisfy 
all claims for omission would result in models of endless and 
labyrinthine complexity. We would like to know if these effects are 
offset by competing processes and if the net effect is of sufficient 
size to alter our conclusions regarding the fossil fuel signal. 

Keeling (1983) has recently examined this problem. He has compared 
measurements of atmospheric C0 2 from a time series of samples 
obtained on board ships from north-south sections in the Pacific 
Ocean. Recent seasonal coverage has greatly increased the utility of 
these data. By assuming that the seasonal oscillations observed today 
hold true for the recent past, we can greatly improve the interpretation 
of previously obtained shipboard data. Figure 3.9 shows plots of such 
data grouped from the periods near 1962, 1968, and 1980 from Keeling 
(1983). The data are normalized relative to a constant value at the 
South Pole, shown as zero on this figure. The peak, centered at the 
equator, is attributed to degassing of C0 2 from the high pCO 2 zone 
in the equatorial Pacific Ocean (Keeling, -1968). This is a natural 
phenomenon. In the northern hemisphere a steady increase has occurred 
that is consistent with rising fossil fuel usage, over 90% of which 



208 



WESTERN TTD 

W W W 2040^ 5 S S 



r^ 



1950 



is s a 



*2IOO 



TOTAL COp 

a ! 




5000 



BOOO L - 



20N 



30N 



A ON 



50'N 



BON 



70N 



FIGURE 3.8 The distribution of total carbon dioxide in seawater along 
the section shown in Figure 3.7. The large natural variability seen 
here must have been perturbed by the invasion of fossil fuel C0 2 , 
although we have no time series of measurements to document this 
change. (From Brewer, 1983.) 



takes place in the northern hemisphere. By assuming that the change in 
atmospheric CC>2 is due solely to fossil fuel usage, with a constant 
oceanic uptake over this period, then model profiles can be developed 
that may be compared with the observed data. Figure 3.10, again from 
Keeling (1983) , shows the result of subtraction of the model profiles 
from the observations. The residual signal reflects the expected 
constant equatorial degassing but does not indicate any significant 
perturbation of atmospheric C02 from other than the fossil fuel 
source. These data, obtained over approximately a 20-year period, 
appear to be the best available. They support, but do not prove, the 
idea of a constant, or very slowly changing, ocean and terrestrial 
biosphere. The atmosphere integrates the effects of changes in these 
systems, so that if a large change is postulated in one of them, then 
there appears to have been a most fortunate compensation in the other. 

Pearman et al. (1983) have also examined this problem. They 
calculate that the dominantly northern hemisphere input of C0 2 to the 
atmosphere has changed the inter hemispheric difference in atmospheric 



209 




40 S 



20 S 



20 N 



40 N 



90 N 



LATITUDE (deg) 



FIGURE 3.9 Atmospheric C(>2 levels along a Pacific Ocean transect , 
normalized to zero at the South Pole. (Prom Keeling, 1983.) 



concentration (north-south) from -1 ppm in the last century to +4-5 ppm 
today* Their atmosphere-ocean model simulation points to a net upper 
limit of 2 Gt of C per year from a terrestrial carbon source. 

The conclusion of Oeschger et al. (1975) was that the proportioning 
of fossil fuel CO 2 between atmosphere and ocean was 60 to 40%. 
Stuiver (1978) concluded that some 47% of fossil fuel CO 2 was stored 
in the ocean. Broecker et al. (1979) calculated that 37 4% of the 
fossil fuel C0 2 generated between 1958 and the present has been taken 
up by the sea. Bolin et al. (1982) have recently concluded that the 
oceanic uptake falls in the range 30-38%. Since these estimates all 
fall in a narrow range, depend on accepted physical principles, and are 
verifiable by several different means, there does not appear to be the 
chance of significant error. 



3.2.10 Future Studies and Problems 

Although present-day models appear satisfactorily to answer the 
question of how much fossil fuel CO 2 the sea takes up, there are many 
things that we do not know. 



210 



2 \ 



a 

i" 




-1 

-2 H 

90 60 50 40 30 



20 10 10 
LATITUDE 



I I I I ! .II 
20 30 40 50 60 90 

N 



FIGURE 3.10 The results shown in Figure 3.9 corrected for seasonal and 
fossil fuel CO 2 effects alone. -No perturbation of the signal from 
other than these effects is apparent. (From Keeling, 1983.) 



We do not know the atmospheric or oceanic C(>2 levels in the last 
century. Recent ice core data (Neftel et al., 1982) promise to place 
new constraints on this. Calculations based on oceanic measurements 
(Brewer 1978 , 1983) yield a preindustrial value of about 265 ppm of 
CO 2 . However, this is likely a lower limit since oceanic deep waters 
are formed in areas of marked negative disequilibrium with the 
atmosphere. As our ocean data base grows, the one-dimensional models 
will become increasingly inadequate, and incorporation of the C0 2 and 
tracer data into new models will be required. 

The warming accompanying the atmospheric CO 2 rise will affect the 
ocean. Storage of heat in the upper layers will mitigate, but not 
prevent, climate change (Bryan et al., 1982). Models of this heat 
storage currently treat it as passive uptake, not affecting water mass 
formation and vertical circulation. If such changes, however, did 
occur, it would affect ocean C(>2 uptake in unknown ways. 

The warming of the ocean will reduce CO 2 solubility and expel 
further C0 2 to the atmosphere. This effect can be calculated 
reasonably well (Bacastow and Keeling, 1979) and is incorporated into 
current models (Killough and Emanuel, 1981) of ocean C0 2 uptake. 



211 

Whether we will detect this effect and separate it in our budgeting of 
C02 between atmosphere, ocean, and biosphere, or confuse it with some 
other signal, is problematic. 

3.2.11 Summary 

On average, each year the ocean currently takes up an amount of CO 2 
approximately equal to 40% of the fossil fuel CO 2 added to the 
atmosphere by man. Calculations based on sound principles of solution 
chemistry and an estimated atmospheric C0 2 content of about 270 ppm 
in the last century show that ocean surface pH must have decreased by 
about 0.06 pH unit during this time; the total CO 2 concentration of 
surface seawater must have increased by about 35 ymol/kg (about 
1.8%). The proportion of carbonate ion must have decreased by about 
10%, increasing the tendency for calcium carbonate dissolution. The 
alkalinity is believed not to have changed. We do not know this on the 
basis of a direct time series of measurements, and until quite 
recently, oceanic C0 2 system measurements contained substantial 
inaccuracies. 

Models of ocean C0 2 uptake have depended greatly on tracer data, 
particularly natural and bomb-produced 14 C. These models represent 
some features of ocean chemistry quite well, but they represent ocean 
physics by a simple vertical diffusion coefficient. The models treat 
only the C0 2 perturbation and do not yet attempt to mimic the natural 
and complex C0 2 -oxygen-nutrient cycles within the ocean. Rapid 
progress in incorporating these features into more advanced ocean 
circulation models is anticipated. 

It is quite possible to measure the changing C0 2 properties of the 
ocean with time using modern techniques, although no ongoing program 
yet exists to do so. There are some significant uncertainties awaiting. 
We do not know when and where calcium carbonate dissolution will occur. 
We do not know how future warming will affect ocean circulation and 
whether we will detect this warming in the C0 2 signal. We do not 
know adequately the time history of CO 2 releases from the biosphere 
so as firmly to close the atmosphere-ocean-biosphere triangle 
represented in our models. 

Finally, we should not be too complacent. Nature has vast resources 
with which to fool us; the last glaciation was apparently accompanied 
by massive CO 2 transfers to and from the ocean, the causes, conse- 
quences, and explanations of which are poorly understood today 
(Broecker, 1982) . 



References 

Aller, R. C. (1982). Carbonate dissolution in nearshore terrigenous 
muds: the role of physical and biological reworking. J. Geol. 
0:79-95. 

Andersen, N. R., and A. Malahoff, eds. (1977). The Fate of Fossil Fuel 
CO-2 in the Oceans. Plenum, New York, 749 pp. 



212 

Bacastow, R. B., and C. D. Keeling (1979). Models to predict future 
atmospheric C0 2 concentrations. In Workshop on the Global Effects 
of Carbon Dioxide from Fossil Fuels. U.S. Dept. of Energy Report 
CONF-770385, pp. 72-90. 

Baes, C. F. (1982) . Ocean Chemistry and Biology. In Carbon Dioxide 
Review; 1982, W. C. Clark, ed. Oxford U. Press, New York, pp. 
187-211. 

Bolin, B. (1981). Carbon Cycle Modelling. SCOPE Report 16. Wiley, 

New York. 
Bolin, B., and C. D. Keeling (1963). Large-scale atmospheric mixing as 

deduced from the seasonal and meridional variations of carbon 

dioxide . J. Geophys. Res. 68; 3899-3920 . 
Bolin, B., E. T. Degens, S. Kempe, and P. Ketner (1979). The Global 

Carbon Cycle. SCOPE Report 13. Wiley , New York, pp. 1-491. 
Bolin, B., A. Bjorkstrom, K. Holmen, and B. Moore (1982). The 

simultaneous use of tracers for ocean circulation studies. Report 

CM-58, Dept. of Meteorology, U. of Stockholm, p. 70. 
Bradshaw, A. L., P. G. Brewer, D. K. Shafer, and R. T. Williams 

(1981) . Measurements of total carbon dioxide and alkalinity by 

potent iome trie titration in the GEOSECS program. Earth Planet. Sci. 

Lett. 55; 99-115. : 

Brewer, P. G. (1978) . Direct observation of the oceanic C0 2 

increase . Geophys. Res. Lett. 5:997-1000 . 
Brewer, P. G. (1983) . The T.T.O. North Atlantic Study A Progress 

Report. In Proceedings; Carbon Dioxide Research Conference; 

Carbon Dioxide, Science and Consensus. U.S. Dept. of Energy Report 

CONF-820970, Part II, pp. 91-122. 
Brewer, P. G., and J. C. Goldman (1976). Alkalinity changes generated 

by phytoplankton growth. Limnol. Oceanog. 21; 108-117. 
Broecker, H. C., J. Peterman, and W. Siems (1978). The influence of 

wind on CO 2 -exchange in a wind-wave tunnel, including the effects 

of monolayers. J. Mar. Res. 36; 595-610. 
Broecker, W. S. (1982). Ocean chemistry during glacial time. Geochim. 

Cosmochim. Acta 46;1689-1705. 
Broecker, W. S., and T. H. Peng (1974). Gas exchange rates between air 

and sea. Tellus 26; 21-35. 
Broecker, W. S., and T. Takahashi (1977). Neutralization of fossil 

fuel C0 2 by marine calcium carbonate. In The Fate of Fossil Fuel 

C0 ? in the Oceans. N. E. Andersen and A. Malahoff , eds. Plenum, 

New York, pp. 213-241. 
Broecker, W. S., T Takahashi, H. J. Simpson, and T. H. Peng (1979). 

Fate of fossil fuel carbon dioxide and the global carbon budget. 

Science 206; 409-418. 
Bryan, K., F. G. Komro, S. Manabe, and M. J. Spelman (1982). Transient 

climate response to increasing atmospheric carbon dioxide. Science 

215: 56-58. 

Bunker, A. F., and I*. V. Worthington (1976). Energy exchange charts of 
the North Atlantic Ocean. Bull. Am. Meteorol. Soc. 57; 670-678. 

Callendar, G. S. (1938), The artificial production of carbon dioxide 
and its influence on temperature. }. J. Roy. Meteorol. Soc. 
4; 223-240. 



213 

Craig, H. (1957). The natural distribution of radiocarbon and the 

exchange time of carbon dioxide between atmosphere and sea. Tellus 

9.: 1-17. 
Emerson, S. R., and M. L. Bender, 1981. Carbon fluxes at the 

sediment-water interface of the deep sea: calcium carbonate 

preservation. J. Mar. Res. 39:139-162. 
Garrels, R. M. , and F. T. Mackenzie, 1981. Some aspects of the role of 

the shallow ocean in global carbon dioxide uptake. U.S. Dept. of 

Energy Report CONF-8003115. 
Garrett, C. (1979). Mixing in the ocean interior. Dynamics Atmos. 

Oceans 3: 239-265 . 
GEOSECS Atlases Vols. 1-6. U.S. Government Printing Office, 

Washington, D.C. 
Hansson, I. (1973). A new set of acidity constants for carbonic acid 

and boric acid in sea water. Deep-Sea Res. 20:461-178. 
Holland, W. R. (1971). Ocean tracer distributions. Tellus 23:371-392. 
Holland, W. R. (1978) . The role of mesoscale eddies in the general 

circulation of the ocean numerical experiments using a wind-driven 

quasi-geostrophic model. J. Phys. Oceanog. 8:363-392. 
Huizenga, D. L., and D. R. Kester (1979). Protonation equilibria of 

marine dissolved organic matter. Limnol. Oceanog. 24:145-150. 
Jenkins, W. J. (1980). Tritium and 3 He in the Sargasso Sea, J. Mar. 

Res. 38:533-569. 
Keeling, C. D. (1968). Carbon dioxide in surface oceans waters, 4. 

Global distribution. J. Geophys. Res. 73:4543-4553. 
Keeling, C. D. (1983). The Global Carbon Cycle: What we know and 

could know from atmospheric, biospheric and oceanic observations. 

In Proceedings; Carbon Dioxide Research Conference; Carbon 

Dioxide, Science and Consensus. U.S. Dept. of Energy Report 

CONF-820970, Part II, pp. 1-62. 
Keeling, C. D., and B. Bolin (1967). The simultaneous use of chemical 

tracers in oceanic studies, I. General theory of reservoir models. 

Tellus 19:566-581. 
Keeling, C. D., and B. Bolin (1968). The simultaneous use of chemical 

tracers in oceanic studies, II. A three reservoir model of the 

North and South Pacific Oceans. Tellus 20:17-54. 
Kempe, S. (1982). Long-term records of CO 2 pressure fluctuations in 

fresh waters. In Transport of Carbon and Minerals in Major World 

Rivers. Mitt. Geol.-Palaont. Inst. Univ. Hamburg, Federal Republic 

of Germany. SCOPE Report 52, pp. 91-332. 
Ki Hough, G. G., and W. R. Emanuel (1981). A comparison of several 

models of carbon turnover in the ocean with respect to their 

distributions of transit time and age responses to atmospheric CC>2 

and 14 C. Tellus 33:274-290. 
Lyman, J. (1956). Buffer mechanism of sea water. Ph.D. Thesis, 

University of California, Los Angeles, 196 pp. 
Mehrbach, C., C. H. Culberson, J. E. Hawley, and R. M. Pytkowicz 

(1973) . Measurement of the apparent dissociation constants of 

carbonic acid in sea water at atmospheric pressure. Limnol. 

Oceanog. 18: 897-907 . 



214 

Millero, F. J. (1979) . The thermodynamics of the carbonate system in 

sea water. Geochim. Cosmochim. Acta 43; 1651-1661 . 
Miyake, Y., Y. Sugimura, and K. Saruhashi (1974). The carbon dioxide 

content in the surface waters of the Pacific Ocean. Rec. Oceanog. 

Works in Jpn. 12; 4 5-5 2. 
Morse, J. W., A. Mucci, and P. J. Millero (1980). The solubility of 

aragonite and calcite in sea water of 35/oo salinity at 25C and 

atmospheric pressure. Geochim. Cosmochim. Acta 44; 85-94 . 
Murray, C. N., and J. P. Riley (1971). The solubility of gases in 

distilled water and sea water, IV. Carbon dioxide. Deep-Sea Res. 

18.: 533-541. 
Neftel, A., H. Oeschger, J. Schwander, B. Stauffer, and R. Zumbrunn 

(1982) . New measurements on ice core samples to determine the C0 2 

content of the atmosphere during the last 40,000 years. Nature 

295:220-223. 
Oeschger, H., U. Siegenthaler, U. Schotterer, and A. Gugelmann (1975). 

A box diffusion model to study the carbon dioxide exchange in 

nature. Tellus 27:168-192. 
Ostlund, H. G. (1983) . Tritium and Radiocarbon: TTO Western North 

Atlantic Section GEOSECS Re-occupation. Tritium Laboratory Data 

Release 83-07, Rosenstiel School of Marine and Atmospheric Sciences, 

Miami, Fla., unpublished data. 
Ostlund, H. G., H. G. Dorsey, and C. G. Rooth (1974). GEOSECS North 

Atlantic radiocarbon and tritium results. Earth Planet. Sci. Lett. 

^3:69-86. 
PCODF (1981). TTO Preliminary Hydrographic Data Reports, Vol. I-IV. 

Scripps Institution of Oceanography Reports, La Jolla, Calif. 
Pearman, G. I., P. Hyson, and P. J. Fraser (1983). The global 

distribution of atmospheric carbon dioxide: 1. Aspects of 

observations and modelling. J. Geophys. Res. 88:3581-3590. 
Peterson, B. J. (1980) . Aquatic primary productivity and the 

14 C-C02 method: a history of the productivity problem. Ann* 

Rev. Ecol. Syst. 11:359-385. 

Reid, J. L., and R. J. Lynn (1971). On the influence of the Norwegian- 
Greenland and Weddell seas upon the bottom waters of the Indian and 

Pacific Oceans. Deep-Sea Res. 18; 1063-1088 
Revelle, R. , and H. E. Suess (1957). Carbon dioxide exchange between 

atmosphere and ocean and the question of an increase of atmospheric 

O>2 during past decades. Tellus 9:18-27. 
Sarmiento, J. L. (1982) . A simulation of bomb tritium entry into the 

Atlantic Ocean. J. Phys. Oceanog., in press. 
Sayles, F. L. (1981) . The composition and diagenesis of interstitial 

solutions, II. Fluxes and diagenesis at the water-sediment 

interface in the high latitude North and South Atlantic. Geochim. 

Cosmochim. Acta 45: 1061-1086 . 
Siegenthaler, U. (1983). Uptake of excess CO 2 by an 

outcrop-diffusion model of the ocean. J. Geophvs. Res. 88:3599-36084 
Skirrow, G. (1975) . The dissolved gases-carbon dioxde. In Chemical 

Oceanography, Vol. 2, 2nd ed., J. P. Riley and G. Skirrow, eds. 

Academic, New York, pp. 1-192. 



215 

Stuiver, M. (1978). Atmospheric carbon dioxide and carbon reservoir 
changes. Science 199:253-258. 

Stuiver, M., P. D. Quay, and H. G. Ostlund (1983). Abyssal water 
carbon-14 distribution and the age of the world oceans. Science 
J219: 849-851. 

Sundquist, E. T., L. N. Plummer, and T. M. L. Wigley (1979). Carbon 
dioxide in the ocean surface: the homogeneous buffer factor. 
Science 204; 1203-1205. 

Takahashi, T. (1979). Carbon dioxide chemistry in ocean water. In 
Workshop on the Global Effects of Carbon Dioxide from Fossil Fuels. 
U.S. Dept. of Energy Report CONF-770385, pp. 63-71. 

Takahashi, T., and A. G. E. Azevedo (1982). The oceans as a CO 2 

reservoir. In Interpretation of Climate and Photochemical Models, 
Ozone and Temperature Measurements, R. A. Beck and J. R. Hummel, 
eds. American Institute of Physics Conf. Proc. No. 82, pp. 83-109. 

Takahashi, T., w. S. Broecker, A. E. Bainbridge, and R. F. Weiss 

(1980a). Carbonate chemistry of the Atlantic, Pacific and Indian 
Oceans: The results of the GEOSECS Expeditions, 1972-1978. Report 
1, cv-1-80.' Lamont-Doherty Geological Observatory. 

Takahashi, T., W. S. Broecker, A. E. Bainbridge, and R. P. Weiss 
(1980b) . Carbonate chemistry of the surface waters of the world 
oceans. In Isotope Marine Chemistry , E. Goldberg, Y. Horibe, and K. 
Saruhashi, eds. Uchida Rokakuho, Tokyo, pp. 147-182. 

Takahashi, T., W. S. Broecker, and A. E. Bainbridge (1981). The 
alkalinity and total carbon dioxide concentration in the world 
oceans. In Carbon Cycle Modelling . B. Bolin, ed. SCOPE Report 16. 
Wiley, New York, pp. 159-199. 

Takahashi, T., D. Chipman, and T. Volk (1983). Geographical, seasonal, 
and secular variations of the partial pressure of C0 2 in surface 
waters of the North Atlantic Ocean: The results of the North 
Atlantic TTO Program, In Proceedings; Carbon Dioxide Research 
Conference; Carbon Dioxide, Science and Consensus. U.S. Dept. of 
Energy Report CONF-820970, Part II, pp. 123-145. 

Walsh, J. J. (1981). A carbon budget for over-fishing off Peru. 
Nature 290: 300-304 . 

Walsh, J. J., G. T. Rowe, R. L. Iverson, and C. P. McRoy (1981). 
Biological export of shelf carbon is a sink of the global C0 2 
cycle. Nature 291:196-201. 

Weiss, R. F. (1974). Carbon dioxide in water and sea water: the 
solubility of a non-ideal gas. Mar. Chem. 2; 203-215 . 



216 

3.3 BIOTIC EFFECTS ON THE CONCENTRATION OF ATMOSPHERIC CARBON 
DIOXIDE: A REVIEW AND PROJECTION 
George M. Woodwell 

3.3.1 Introduction 

The composition of the atmosphere is changing. It has changed greatly, 
of course f throughout the period of the Earth's evolution. Few doubt 
the dominant role of the biota in the evolution of the atmosphere: the 
oxygen is residual from storage in the crust of reduced carbon com- 
pounds fixed by plants over hundreds of millions of years. The current 
changes are due in part to mobilization of these fossil reserves as 
fuel, as well as to changes in the amount of carbon retained in the 
biota and soils globally. The immediate question is how large the 
recent and future influence of the biota may be. 

The primary evidence for current change is the record of observations 
of the C02 content of the atmosphere. Modern records show not only a 
year-by-year increase of between about 0.5 and 2 ppm in C02 annually 
but also a seasonal fluctuation. The amplitude of the oscillation 
varies with latitude, altitude, and probably with other factors (see 
Machta, Section 3.4). The amplitude approaches 20 ppm in central Long 
Island (Woodwell et al., 1973a) and in Barrow, Alaska (Kelley, 1969). 
It is about 5 ppm at Mauna Loa and 1 ppm at the South Pole (Keeling et 
al., 1976a,b) . The peak C0 2 concentration occurs in late winter; the 
minimum occurs in early fall. The oscillation is reversed in the 
southern hemisphere to coincide with the southern seasons. These 
observations are evidence that biotic factors are large enough to 
influence the C0 2 content of the atmosphere in the short term over 
large regions, possibly the Earth as a whole. We know, in addition, 
that the terrestrial biota and soils contain two to three times as much 
carbon as the atmosphere. A small change in the size of these reser- 
voirs globally has the potential for storing or releasing a quantity of 
C0 2 sufficient to affect the amount in the atmosphere appreciably. 

Despite the strength of these observations, answers to specific 
questions about the role of the biota have proven elusive. Can we use 
the history of the terrestrial biota over the past century to help 
determine the preindustrial atmospheric C02 concentration (and thus 
the sensitivity of climate to increasing C0 2 )? What fraction of the 
increase in C0 2 observed since 1958 at Mauna Loa is due to oxidation 
of carbon compounds in plants and soils as opposed to combustion of 
fossil fuels? What proportion of future C0 2 emissions will be 
absorbed into biotic reservoirs? If current trends in land use con- 
tinue, what effect will they have on future atmospheric concentrations? 
To what extent could the future C02 content of the atmosphere be 
controlled by management of land and forests? 

In the sections that follow we discuss the factors that affect the 
role of the biota in determining atmospheric C(>2 concentrations. 
These factors include the size and location of major reservoirs of 
carbon, the various transitions in metabolism that affect the 
reservoirs, and direct human effects, such as the clearing of forested 
land for agriculture. 



217 
3.3.2 How Much Carbon is Held in the Biota and Soils? 

3.3.2.1 The Biota 

Over the past decade , the most widely used appraisal of the global 
carbon content of biotic systems has been that of Whittaker and Likens 
(1973). Whittaker and Likens estimated that the world of 1950 held in 
the biota a total of 829 gigatons of carbon (Gt of C) , almost entirely 
on land. By far the largest biotic reservoir, 743 Gt of C, was 
estimated to be in forests. Forests have been reduced in area in the 
more than 30 years since 1950. More recent estimates (e.g./ Ajtay et 
al., 1979) of terrestrial biomass have suggested additionally that the 
average standing crop of organic matter per unit area is lower than 
estimated by Whittaker and Likens (1973) . The most comprehensive 
recent analysis is that of Olson (1982) 9 who has suggested a total 
biomass for 1980 of 560 Gt of C. The differences among these estimates 
are probably due largely to differences in interpretation and in 
assumptions; they may be due in part to reduction in the area of 
forests between 1950 and 1980. There is no easy resolution. To the 
extent that areas in forests are in question/ satellite imagery will 
offer greatly improved estimates. Better data on biomass will require 
additional field studies. 

3.3.2.2 The Soils 

The total carbon retained in soils has been estimated globally as 
between about 1400 and 3000 Gt (Table 3.1). Schlesinger (1977/ 1983) 
has reviewed the estimates and has suggested that there is a total of 
about 1500 Gt of readily mobilized carbon available. This conclusion 
has been supported recently by Post et al. (1982) / who summarized data 
from more than 2000 samples of soils from around the world. 

3.3.2.3 Total Carbon Pool under Biotic Influences 

The total amount of carbon readily transformable into C0 2 by metabolic 
processes is probably in the range of 2000-2500 Gt, about 3 times the 
725 Gt of C currently held in the atmosphere. Most of this inventory 
is associated with forests. 

The fraction of the biotic pool that will actually be transformed to 
atmospheric CO 2 in future decades is speculative. In principle/ a 
large portion could be. Evidence is that when forests are disturbed or 
replaced by agriculture there is a substantial loss of carbon. Loss 
from the original standing stock of plants usually exceeds 90%, and 
loss from soils can also be substantial. Plough horizons of soils long 
in agriculture commonly contain 1-3% carbon or less on a total dry 
weight basis. Original soils of the forests from which the 
agricultural soils were developed may have contained as much as 40-50% 
carbon in the surface horizon. Total loss over the entire profile is 
probably 20-50% of the original amount in the soil. 



218 

TABLE 3.1 Amount of Carbon Retained in Soils Globally According to 
Various Recent Estimates* 



Total Carbon 

in Soils 

of the Earth 

(Gt) Source 

3000 Bohn, 1978 

2070 Ajtay et al. f 1979 

1456 Schlesinger, 1977 

1395 Post et al., 1982 

1477 Buringh (in press , 1983) 



data of Post et al. (1982) are the most comprehensive and recent 
estimates and are based on more than 2000 samples from around the world. 



3.3.3 Metabolism and the Storage of Carbon in Terrestrial and Aquatic 
Ecosystems 

3.3.3.1 The Production Equation 

The net flux of carbon between the atmosphere and any ecosystem is 
determined by the balance between gross photosynthesis and total 
respiration. The relationship is shown by the production equation for 
an ecosystem (Woodwell and Whittaker, 1968): 

NEP = QP - (R A + %), 

where NEP is the net ecosystem production, the net flux of carbon into 
or from an ecosystem; GP is the gross production, total photosynthesis 
of the ecosystem; R A is the respiration of the autotrophs, the green 
plants; % is the respiration of the heterotrophs, including all 
animals and organisms of decay; and R^ + BJJ is the total 
respiration of the ecosystem. 

The potential range of values for NEP is from a large negative 
number, indicating a loss of stored carbon, to a positive value that 
approaches the net amount of carbon available from green plants after 
their own needs for respiration (R A ) have been met. This excess 
above respiration is commonly called net primary production (NP) . Its 
relationship to gross production is given by the equation 

NP m GP - R A , 

where GP is the total photosynthesis of the ecosystem, as above; RA 
is the respiration of the autotrophs; and NEP is NP - R H . 

The production equations have been especially useful in analyses of 
the metabolism and carbon flux of forests, where the terms can be 



219 



Biomass and soil 
carbon (g/m ) 




YEARS 

FIGURE 3.11 Relationship between net ecosystem production (NEP) and 
the total accumulation of carbon in a forest. 



evaluated conveniently (for example/ see Woodwell and Whittaker, 1968; 
Whittaker and Woodwell, 1969; Woodwell and Botkin, 1970; Reichle et 
al., 1973) The equations are applicable in aquatic systems as well 
(Woodwell et al., 1973b, 1979). 

Net ecosystem production varies from zero at the start of the 
successional development of a forest to a maximum at midsuccession and 
back to zero at climax (Figure 3.11). The relationship emphasizes that 
undisturbed forests approach an equilibrium (climax) in which gross 
photosynthesis is equaled by total respiration. Any change in the 
relationship between gross production and either segment of the total 
respiration shifts the ratio and causes a positive or negative net 
ecosystem production. In general, photosynthesis is more vulnerable to 
disruption than total respiration. This generalization holds for both 
the individual plant and for the ecosystem as a whole. The reason is 
that photosynthesis is limited to certain plants and is dependent on 
many factors, all of which must be favorable; respiration is a general 
characteristic of all life and occurs in some form under a wide range 
of conditions. Virtually any disturbance (e.g., forest fire, land 
clearing, air pollution) favors respiration over photosynthesis, at 
least initially (see discussion below) and tends to result in transfer 
of carbon from the biota into the atmosphere. 



3.3.3.2 A Basis in the Metabolism of Forests for the Oscillation in 
Atmospheric C0 2 Concentration 

Our understanding of the metabolism of terrestrial ecosystems, 
including forests, agricultural systems, grasslands, and other 
communities, supports the hypothesis that the annual oscillation in 
C02 is due largely to the metabolism of temperate zone forests. The 
forests dominate because of their size, both in area and in magnitude 
of their metabolism. The annual course of metabolism of a temperate 
zone forest, taken in toto on a unit of land, expressed as gross 
photosynthesis and total respiration in separate curves, appears in 
Figure 3. 12 (a). Net ecosystem production at any time is the algebraic 



220 



FACTORS AFFECTING CO 2 IN AIR 



O 

GO 
CC 



LL 
O 
CO 



7 

5 

i 



O 
U. 

O 



QC 

o 

LU 




1360 



- 350 



340 



I 



330 8 



320 



310 



11/76 



MAM 



FIGURE 3.12 (a) The course of total respiration and gross photo- 
synthesis of an oak-pine forest in central Long Island, New York. 
Integration of these two curves produced the prediction of the annual 
change in atmospheric 002 shown in the curve below, (b) The 
amplitude predicted in this way was considerably greater than observed 
(Woodwell et al., 1973a) , apparently because of mixing with air from 
over the oceans. 



221 

sum of one point on each of the two curves. Integrating over the 
entire year produces a curve for net ecosystem production [Figure 
3.12 (b)]. This new curve follows closely the pattern of oscillation 
observed in the C0 2 concentration at Mauna Loa and elsewhere, 
although the amplitude observed around the world differs greatly from 
that calculated for the forest of central Long Island shown in Figure 
3.12. 



3.3.3.3 Factors Affecting Global Net Ecosystem Production 

3.3.3.3.1 Succession and the Equilibrium Hypothesis 

The equilibrium between gross photosynthesis and total respiration 
is achieved in forests after a period of success ional development that 
may last decades to a century or so. It probably occurs in the pelagic 
segment of aquatic systems in days to weeks after disturbance. It is 
approximate: there is a residuum of the annual increment of fixed 
carbon that is stored in sediments both on land and in the sea, but the 
storage is a small fraction of annual production (Broecker, 1974) . To 
illustrate, one of the largest accumulations of stored carbon on land 
is in the peat of the tundra and boreal forest, which we might estimate 
at 500 Gt of C. This mass is believed to have accumulated over the 
10,000 years since the retreat of glacial ice, an annual rate of 
accumulation 0.05 Gt of C. The assumption of an equilibrium seems 
appropriate for this first approximation of the biotic flux. 

Aquatic ecosystems move more rapidly than terrestrial systems toward 
equilibrium between gross photosynthesis and total respiration because 
the species are small-bodied and reproduce rapidly. Mechanical dis- 
turbance is common and has little effect on the ratio of gross produc- 
tion to total respiration under most circumstances. Chemical distur- 
bance in the form of enrichment or pollution may simply speed the rate 
of turnover of carbon molecules, not increase the rate of sedimentation, 
at least through a wide range (Peterson, 1982) , although Walsh et al. 
(1981) have suggested that enrichment of the coastal zone with nitrogen 
and phosphorus is causing increased accumulation of sediments on the 
shelf. 

3.3.3.3.2 Gross Photosynthesis 

Gross photosynthesis is awkward to measure directly in nature. It 
is normally measured as net photosynthesis of leaves plus total 
respiration of the ecosystem. Data are usually taken on a small scale 
in controlled environments or in carefully monitored natural ecosystems. 
Techniques are available now to provide more and better data on patterns 
and trends in the metabolism of ecosystems and should be exploited. 

Several factors affect gross photosynthesis. The most important are 
light, moisture, and availability of nutrients, especially nitrogen, 
phosphorus, and CO 2 . While it is common to think of one factor at a 
time as limiting the rate of any process such as photosynthesis, experi- 
ence indicates that, throughout wide ranges, a change in the avail- 



222 

ability of any factor will produce a response. The dominant question 
for our consideration is whether the increase in atmospheric C0 2 is 
causing an increase in net ecosystem production globally. Waggoner 
(this volume, Chapter 6) , Strain (1978) , and a recent symposium (AAAS r 
in press) in Athens, Georgia, have reviewed studies in which efforts 
have been made to measure the effect of increased C0 2 on the growth 
of crop plants. While there is probably an important effect on growth 
of well-watered, fertilized plants, there is question as to whether 
these effects extend to natural communities. The following tentative 
generalizations may be offered (Kramer, 1981) : 

* Species differ greatly in response to enhanced C0 2 . 

The response is greater in plants with indeterminate growth 
(cotton) than in plants with determinate growth (corn) . 

The response is greater in 3 plants such as soybeans than in 
4 plants such as corn. 

The largest response is in seedlings; in older plants the 
response decreases or ceases. 

One of the most important factors is that any increase in growth 
observed is not always due to an increase in the rate of photosynthesis 
per unit of leaf area. Enhanced CO 2 concentrations cause changes in 
the morphology of growing plants, including an increase in branching of 
both woody and herbaceous plants, greater stem elongation, and an 
increase in the ratio between roots and shoots. One of the most per- 
sistent effects is an increase in the area of leaves, a result observed 
by Wong (1979) in a series of studies of effects of nitrogen nutrition 
and CO 2 on cotton and by others in several studies summarized by 
Kramer (1981) and Strain (1978) . 

While Waggoner (this volume. Chapter 6) reports generally beneficial 
effects on growth of crop plants, a positive response to increased CO 2 
is not universal. Wong (1979) showed that corn plants assimilated less 
carbon under high CO 2 and speculated that contrary observations in 
other experiments may have been due to lower light intensities. 
Responses to enhanced CO 2 concentrations are particularly strong in 
younger plants, a factor that affects many of the experiments reported/ 
such as that by Gif ford (1979) , in which a stimulation of growth was 
shown in wheat grown under moisture stress with enhanced CO 2 concen- 
tration. In longer-term experiments and in older plants response is 
diminished, disappears, or may involve a reduction in growth rate. 
These considerations should lead to caution in projecting effects of 
enhanced concentrations of C0 2 on photosynthesis in forests (Strain, 
1978; Kramer, 1981) . 

Despite the expansion of agriculture, natural (unmanaged) forests 
still dominate the biotic segment of the global carbon cycle. Plants 
in natural forests live in conditions of extreme competition for light, 
water, nutrients, space, and, probably, CO 2 during daylight. None of 
the research reported applies directly to this circumstance, a fact 
that suggests great caution in predicting enhanced storage of carbon in 
natural systems due to increased atmospheric C0 2 . In fact, Kramer 
(1981) concluded that in general, increase in C0 2 concentration will 



223 

probably have the least effect on growth of plants in closed stands 
where light f water , and mineral nutrition, separately or collectively , 
already limit the rate of photosynthesis. Kramer's conclusion is 
supported by observations that the rate of photosynthesis per unit leaf 
area is not always increased by an increase in CC>2 concentration and 
that effects on plants often involve a change in the morphology in 
young plants. Forests are not modified rapidly in the latter respect . 
Effects of temperature on gross photosynthesis are through effects 
on respiration, or other processes apart from the photochemical 
process, which is nearly independent of temperature (Larcher, 1980, p. 
111). 

3.3.3.3.3 Total Respiration 

The data on respiration that are of significance in detecting a 
change in net ecosystem production are those that define the rate per 
unit of land area. They include the respiration of the community of 
higher plants, the community of animals, and the various communities of 
lichens, mosses, and organisms of decay* Such data have rarely been 
taken for terrestrial ecosystems; analysis must be based largely on 
inference from first principles or from data obtained for other 
purposes . 

Rates of respiration are also affected by many factors, including 
availability of water, nutrients, especially nitrogen and phosphorus, 
and temperature. As in all chemical reactions, rates are affected by 
the availability of substrates and the accumulation of products. The 
greatest sensitivity is probably to temperature. A 10C increase in 
temperature through the middle range of the response curve for a whole 
plant commonly produces a twofold to threefold increase in the rate of 
respiration (Table 3.2) . Experimental evidence from tundra communities 
confirms the effect of warming (Billings et al., 1982). By comparison, 
the effects of other factors appear small. Direct effects of CO 2 
concentrations in the range of 300-600 ppm on total respiration of an 
ecosystem are so small as to remain unmeasured and probably unmeasure- 
able. The observation that rates of respiration in photosynthetic 
tissues differ in the light and in the dark (Zelitch, 1971) has little 
bearing on the total respiration, so large is this total relative to 
the fraction that occurs in photosynthetic tissues. 

3.3.3.3.4 Net Ecosystem Production and the 6 -Factor 

In an effort to resolve a discrepancy between the amount of carbon 
reportedly released through combustion of fossil fuels and the amount 
apparently transferred to the oceans, Bacastow and Keeling (1973) 
introduced a factor into their analysis that allowed an expansion of 
the biotic pool of carbon as a function of the increase in CO 2 in 
air. It was assumed that this so-called "3 -factor" was the only 
important biotic consideration in the global carbon cycle. P was 
estimated to be a constant with a value of 0.26. The P -factor as 
formulated by Bacastow and Keeling was limited to the putatively 
positive effect of the increase in CO 2 on net ecosystem production. 



224 

TABLE 3.2 Respiratory Quotients (Q 10 ) for Plants and Plant 
Communities^. 



Zone/Species 


Respiratory Temperature 
Q 10 Range (C) 


Reference 


Pea seedlings 


3.0 


0-10 


Giese, 1968 




2.4 


10-20 






1.8 


20-30 






1.4 


30-40 




Plants 


2.1-2.6 


0-30 


Fitter and Hay, 1981 


Tundra/Taiga 


2 


Low temperature 


Miller, 1981 




2 


High temperature 




Tropics 


3 


10 


Larcher, 1980 


Clover 


2.4 


3-13 


Woledge and Dennis, 1982 


Greenland plants 


2.0-2.7 


0-25 


Eckhardt et al., 1982 



is the factor by which respiration is increased by a 10C 
increase in temperature. 



No consideration was given the possibility that processes other than 
002 enrichment might affect the amount of carbon retained in the 
biota and soils or the possibility that the area of forests might be 
changing globally. The use of the B -factor was a pragmatic solution 
to a complex and puzzling issue that arose from attempts to analyze the 
global carbon cycle through a simple model. Its use should now be 
replaced by separate analyses of the effects of (a) changes in the area 
of forests and (b) potential changes in net ecosystem production caused 
by both increased atmospheric CO 2 and changes in climate. The latter 
will require modeling based on processes in terrestrial ecosystems. 

The 002 content of the atmosphere has increased since 1860 to its 
current 340 ppm from a concentration now estimated at 260-280 ppm (see 
Machta, Section 3.4). The increase may be approaching 30%. Such a 
change in 002 content alone has probably had no effect on respiration. 
It may have affected gross photosynthesis, but if so, the change is 
detectable neither directly as a measurement of net or gross photo- 
synthesis nor indirectly as a measurement of some segment of net 
ecosystem production such as the annual increment of wood in trees of 
seasonal forests. A widespread increase in annual tree growth of as 
little as 10% should be detectable as a universal or very common change 
in width of tree rings. No such stimulation is conspicuous. Rebello 
and Wagener (1976) found evidence in Europe of an increase in diameter 
growth, but Whittaker et al. (1974) found the opposite in North America. 
A smaller increase might remain undetected at present. Waggoner (this 
volume, Chapter 6) suggests a small C15%) increase for crop plants 
in a 400-ppm atmosphere. 

While some atmospheric changes may have favored growth in the total 
carbon held in the biota and soils globally, others may not. In par- 



225 

ticular, there is a need to consider the possible role of a long-term 
global warming of about 0.5C since the lows of the late 1880s (see 
Weller et al., this volume, Chapter 5). If a 10C increase in tem- 
perature increases rates of respiration twofold to threefold (Table 
3.2), the 0.5C warming observed may have increased total respiration 
of terrestrial ecosystems by 10-15%. Such a change would appear as a 
reduction in net ecosystem production; it might, of course, simply 
offset an increase in gross photosynthesis. The topic of changes in 
the biota as a result of enhanced CC>2 and climatic change requires 
detailed study through descriptive surveys and careful field experi- 
mentation under controlled circumstances. At the moment there is no 
direct evidence that net ecosystem production has changed per unit area 
of existing forests regionally or globally over the past century. 



3.3.4 Changes in Area of Forests of the World 

There have been many changes in the area of forests in postglacial 
time. The transitions have been caused by climatic changes such as 
those that accompanied the retreat of glacial ice, by shifts in 
patterns of distribution of rainfall, and by activities of man. The 
Levant, for instance, was probably largely deforested 5000 or more 
years ago through harvest of wood followed by intensive and prolonged 
grazing by goats. Other sections of the Mediterranean Basin were 
deforested more recently, but still 1000-5000 years ago. The British 
Isles were largely forested until the seventeenth century. Other 
areas, including much of Europe and northeastern North America, were 
cleared for agriculture and grazing two to three centuries ago. Agri- 
culture was subsequently abandoned in some areas, and these have been 
partially reforested. Overall, the expansion of human population 
throughout history has been accompanied by an almost continuous decline 
in the area of forests globally. 

The rate at which deforestation occurred in the past was slow by 
comparison with recent rates. The changes that affected areas as large 
as the Mediterranean Basin, the Levant, or the British Isles took place 
over centuries to millennia. Rates of CO 2 releases from biotic 
sources probably varied considerably and may have affected the CO 2 
content of the atmosphere significantly, but there is no basis for 
measurements either of the biotic releases or of the CO 2 accumulation 
that followed them. During the past century, higher rates of deforesta- 
tion may have been resulting in annual releases of CO 2 of as much as 
several billions of tons of carbon, in some years increasing the total 
atmospheric burden by as much as 0.5%. 

The amount of the biotic contribution to the atmospheric increase is 
in question. The challenge has been measurement: how can information 
on rates of deforestation, and reforestation whenever it occurs, be 
summed for the Earth as a whole? The greatest uncertainty is in the 
rates of deforestation in the tropics, but there is uncertainty about 
the size of the contribution from loss of temperate zone and boreal 
forests as well. The largest areas of forest remaining in the world 
are in the tropics, especially the Amazon, and in the northern tern- 



226 

TABLE 3.3 Estimates of Annual Net Carbon Plux between Terrestrial 
Ecosystems and the Atmosphere in or about 198 OSL 



Author 10 15 g of C/yr 



Adams et al., 1977 


0.4 to 4 


Bolin, 1977 


0.4 to 1.6 


Revelle and Munk, 1977 


1.6 


Wong, 1978 


1.9 


Woodwell et al., 1978 


4 to 8 


Hampicke, 1979 


1.5 to 4.5 


Seiler and Crutzen, 1980 


-2.0 to 2.0 


Brown and Lugo, 1981 


-1.0 to 0.5 


Moore et al., 1981 


2.2 to 4.7 


Olson, 1982 


0.5 to 2.0 


Houghton et al., 1983 


1.8 to 4.7 



^Positive values indicate net release to the atmosphere. Full 
citations in references. 



perate and boreal zones. There are significant areas of forests 
remaining, however, in tropical Africa and in Southeast Asia. 

There have been numerous attempts over the past decade to estimate 
the current annual net carbon flux between terrestrial ecosystems and 
the atmosphere. Estimates, summarized by Clark et al. (1982) , range 
from a. net absorption of 2.0 Gt of C to a net release of 20 Gt of C per 
year. Table 3.3 presents a selection of recent estimates. All the 
estimates suffer from a lack of persuasive detail as to rates of defor- 
estation in key areas. The estimates differ in large part because they 
have treated different segments of the problem. When corrected to a 
common basis, the recent estimates converge considerably (Woodwell et 
al., 1982) . 

The most important advances in these analyses have come through 
recognition that sufficient information is available to allow 
prediction of the details of changes in forested areas if the time of 
(a) harvest or (b) transformation to agricultural or grazing land is 
known. A forest that is harvested by clearcutting and allowed to 
recover, for example, follows a predictable pattern of successional 
development. The observation that disturbance occurred is the critical 
point: the sequel is predictable. Similarly, a forest transformed to 
pasture or to row-crop agriculture loses its carbon stock predictably. 
If agriculture is abandoned, the forest recovers, again at predictable 
rates. Precision in the total inventory of carbon is less important 
than the evidence of change in the inventory. The evidence of change 
is abrupt and discontinuous, namely, the harvest of a forest or the 
abandonment of agriculture. 

Evidence on changes in land use can be accumulated, tabulated, and 
summed in a model to offer an estimate at any time of the trends in 



227 

carbon storage in forests, locally or globally. Such a model , con- 
structed around the central principle of ecological succession, has 
been developed and used; results are reported below. Details of the 
construction of the model, including the data and assumptions used and 
tests of sensitivity, have been presented elsewhere (Moore et al., 1981; 
Houghton et al. f 1983; Woodwell et al., 1983a) . The model accommodates 
12 geographic regions and 10 different types of vegetation in each 
region. Transitions in soils are also included. A few comments about 
quality and sources of data are necessary. 

Data appraising rates of change in forested areas are surprisingly 
difficult to obtain and verify. The major factors to be measured are 
(1) change in the amount of carbon per unit area of forests and (2) 
change in the area of forests. Neither can be measured unequivocally 
for the globe at present. 

Several factors contribute to the difficulties in obtaining sound 
data. One is that no nation is proud of the destruction of a valued 
resource, and national statistics are often unreliable. In addition, 
many nations lack equipment and personnel needed for gathering data on 
area of forests remaining and for evaluating such data. There is 
always question as to what is forest. Are the successional stands that 
replace moist tropical forests following harvest to be considered 
equivalent to the forests they replaced? Are impoverished, heavily 
grazed woodlands forests? Various economic considerations tend to bias 
reporting first one way, then another (Persson 1974, 1977) . Recent 
studies of remote sensing using the LANDSAT and NOAA systems show that 
satellite imagery offers great promise for improving data on areas of 
forests globally (Woodwell, 1980; Woodwell et al., 1983b; Woodwell et 
al., in press) . 

Three sources for estimating rates of deforestation have been used 
extensively in recent analyses. One source is the series of Production 
Yearbooks of the UN Food and Agriculture Organization (PAO) , published 
since 1949. They rely on data reported by governments. A second source 
is the work of Myers (1980) , who has compiled detailed estimates of the 
rate of conversion of tropical forests using a variety of sources. The 
emphasis on the tropics is appropriate because of the extraordinary 
growth in the human population in the tropics and the surge of economic 
development that has affected the tropical regions since the Second 
World War. A third source of estimation is based on the assumption 
that there is a simple correlation between growth in human population 
and rate of deforestation. The basis of the assumption is that most of 
the loss of forests is due to conversion for agriculture. Revelle and 
Munk (1977) developed this approach initially.* 



*Richards et al. (1983) have recently used historical records to 
determine land area converted from unmanaged ecosystems to regularly 
planted cropland. They estimate net conversion of 851 million hectares 
between 1860-1978; Revelle and Munk estimated clearing between 
1860-1970 of 853 million hectares. 



228 



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229 

Analyses using the three sources have been reported in detail by 
Houghton et al. (1983), and comparisons have been made with other, less 
comprehensive analyses that appeared to reach different conclusions, 
such as those of Detwiler et al. (1981) and Brown and Lugo (1981) . In 
each instance, when appropriate adjustments were made to include all 
major factors, such as decay of soil organic matter, the analyses 
converged to a narrower range. Olson (1982) , on the other hand, 
estimated a net loss of slightly less than +1 Gt of C from the 
terrestrial sources in 1980; the estimate was based on lower appraisals 
of the mass of carbon per unit area than those used by Houghton et al. 
(1983) and interpretations of changes in the area of forests that are 
difficult to document. 

The results of the application of the model using the three sources 
appear in Table 3.4. The range of the total release of carbon from the 
biota and soils into the atmosphere since 1860 varied little, between 
180 and 185 Gt. Most of the difference occurred in recent decades: 
the range of estimates between 1958 and 1980 was 52-70 Gt of C. The 
annual net release in 1980 was estimated as between 1.8 and 4.7 Gt of C. 

The model was used to test assumptions about the d-^ta and the 
significance of various biotic processes. These tests showed that 
lower total releases over 120 years could have occurred if agricultural 
land were taken only from nonforested areas (5b in Table 3.4), if there 
were reduced decay of soil carbon (8 in Table 3.4), if there were very 
much more rapid recovery of disturbed forests than experience dictated 
(10 in Table 3.4), and if there were substantially less carbon in the 
vegetation and soils than sources such as Whittaker and Likens (1973) 
had indicated (11 in Table 3.4). These changes were all applied to the 
population-based estimate. None of these modifications turned the 
biotic pools into a net sink for atmospheric carbon at any time in the 
120-year period. The annual release in 1980 reached a minimum of 1.7 x 
10 Gt of C when the assumption was made that all agricultural land was 
taken from nonforested areas. That assumption must be considered 
unrealistic; it was included in the analysis to provide a limit for 
comparison. 



3.3.5 The Biota in the Context of the Global Carbon Balance 

The effort to this point has been to summarize the most probable 
transitions in the biotic pools of carbon globally over the past 
century. The results are not consistent with current estimates of 
other segments of the global carbon cycle. The global carbon balance 
can be expressed as 

A = F - S + B, 

where A is the increase in the carbon content of the atmosphere over 
any period, F is the release of carbon to the atmosphere from 
combustion of fossil fuels in the same period, 8 is the net transfer to 
the oceans in the same period, and B is the absorption or release of 
carbon by the biota in the same period. 



230 

This equation can be evaluated to estimate the role of the biota. 
For example f for 1980 Houghton et al. (1983) reported (in Gt of C per 
year) 

2.5 (0.2) + 5.2 (+0.7) - 2.0 (+0.5) - 0.7(+1.4). 

The sources of the quantities in this case were A = 2.5 for 1980 from 
Bacastow and Keeling (1981), F = 5.2 for 1980 from Rotty (1981), and 
5 s 2.0 calculated as 39% of the fossil fuel release following Broecker 
et al. (1979) . If the estimates of A, F, and S are accepted, the biota 
absorbed 0.7 Gt of C in 1980. If A, F, and S are stretched to their 
proposed limits of uncertainty, the biota may have absorbed 2.1 Gt of C 
or released 0.7 Gt of C. The method is indirect, but it has the 
advantage of presenting simply the relationships among the various 
major fluxes of carbon. 

Alternatively, methods based on isotopic dilution may be used to 
estimate total biotic effects over a long period of time. Results 
suggest a net release from the biota of between 70 and 195 Gt of C over 
a century or more (Table 3.5). These studies cannot provide precise 
estimates for a given year. 

As we have seen, most direct analyses of recent changes in the 
biotic reservoirs of carbon, when they include releases of carbon from 
decay of organic residues from the plants, organic matter in soils, and 
the decay of wood and forest products removed from the site, show a net 
release of carbon from destruction of forests. The release is estimated 
currently here on the basis of extensive experience as between 1.8 and 
4.7 Gt of C per year (Houghton et al., 1983; Woodwell et al., 1983a) . 
This range is well outside the estimates above, including the ranges of 
uncertainty. Nonetheless, if the actual release is at the lower end of 
this range, there may be some basis for argument that the global carbon 
equation is balanced within the range of uncertainty associated with 
the other terms. If the actual value is near the upper end of the 
range, the equation is unreconc liable. A recent re interpretation of 



TABLE 3.5 Estimates of the Release of Carbon from the Terrestrial 
Biota and Soils to the Atmosphere during the Past Century Based on 
Studies of Isotopes of Carbon in Tree RingsS, 



Reference Period Gt of C 

Stuiver, 1978 1850-1950 120 

Wagener, 1978 1800-1935 170 

Freyer, 1978 1860-1974 70 

Siegenthaler and Oeschger, 1978 1860-1974 135-195 

Tans, 1978 1850-1950 150 

^Adapted from Houghton et al. (1982) . 



231 

the data from Myers (1980) on forest conversion in the tropics suggests 
that the upper limit of this range may be as low as 3,0 Gt of C. 

One hypothesis frequently proposed to balance the carbon equation is 
that net ecosystem production on land has increased in response to the 
increase in CO 2 in the atmosphere. Such an increase would have to 
have been substantial, as much as 100-200 Gt of C over the past 120 
years, to overcome the biotic losses to the atmosphere described above. 
It would probably be detected as a universal increase in diameter 
growth of trees or storage of humus in soils. Any such stimulation of 
carbon storage would have been accelerating as deforestation has 
proceeded, reducing areas where storage could occur. Moreover, it 
would require an increased spread between gross production and total 
respiration globally. A warming trend would probably work counter to 
this by increasing rates of respiration of plants and soil organic 
matter without a corresponding increase in gross production. This 
relationship would persist unless other factors, too, were ameliorated, 
such as water supply and the availability of nutrient elements, 
especially N and P. 

Additional insight on the role of the biota has been sought through 
exploration of the oscillation in CO 2 concentration observed at Mauna 
Loa (Hall et al., 1975; Bacastow et al., 1981b; Pearman and Hyson, 
1981) . The hypothesis is that a change in the area of forests or a 
change in net ecosystem production regionally should be reflected over 
the 25-year record in a systematic change in the amplitude of the 
oscillation. Analyses showed no identifiable trend over the first 15 
years (Hall et al., 1975). There are now indications that the ampli- 
tude has been increasing in recent years (Bacastow et al., 1981a,b; 
Machta, this volume, Chapter 3.4). 

How is such a change in amplitude to be interpreted? There appears 
to be little question that the oscillation itself is caused by the 
metabolism of forests. If the amplitude can be assured to be free of 
potentially confounding effects, it may offer an appraisal of the 
status of net ecosystem production at any moment for a segment of the 
northern hemisphere (Figure 3.12) . Unfortunately, the coupling between 
the oscillation and metabolism of forests is too loose for the amplitude 
to be considered a unique measurement of metabolism. Many factors 
affect it. These include, for example, atmospheric mixing: the ampli- 
tude of the oscillation is reduced at higher elevations and at lower 
latitudes. Small changes in patterns of circulation of air can be 
expected to affect it, as well as variability in the temperature of 
seawater. The amplitude is also open to various direct effects of 
metabolism of forests. A warm winter in the northern hemisphere, for 
example, would increase the total respiration without affecting the 
photosynthetic withdrawal during summer appreciably; the excess CO 2 
would appear as an increased late-winter peak in the Mauna Loa record. 
That C0 2 would be mixed into the rest of the atmosphere over the 
ensuing weeks and would have little or no effect on the late summer 
minimum. The amplitude of the oscillation would have been increased 
and the total biotic pool of carbon on land reduced. The converse, an 
increase in the storage of carbon during summer, whatever the cause, 
would also appear as an increased amplitude in the oscillation. Vari- 



232 

ations in fossil fuel use may also be a confusing factor. While the 
oscillation of the Mauna Loa record may be interpretable, reliance on 
the record to identify transitions in the biota must await considerably 
greater attention to detail and better data than have been available so 
far. 

The possibility remains that aquatic systems have been stimulated in 
some way into accelerated storage of fixed carbon in sediments or in 
the deeper waters of the oceans. Freshwater systems can be ignored: 
they are very small in comparison with the oceans, which cover two 
thirds of the surface of the Earth. Baes (1982) and others (Smith, 
1981; Walsh et al., 1981) have suggested various mechanisms by which 
biotic activity might result in sedimentation of carbon. To be signifi- 
cant in the global cycle these mechanisms would have to account for 1 
Gt of C or more annually and would have to respond in some way roughly 
proportional to the increase in C02 in the atmosphere. While there 
is no question about the capacity of the oceanic biota, either in 
coastal areas (Walsh et al., 1981) or in the open oceans (Baes, 1982) 
to fix sufficient carbon to be significant in the global balance, there 
is question as to whether enough fixed carbon is sequestered in these 
waters to affect the global cycle in ways measurable now on a year-by- 
year basis. In pelagic aquatic systems gross production and total 
respiration tend to be closely coupled. An increase in photosynthesis 
is quickly followed by an increase in respiration; storage in sediments 
is small. 

Peterson (1982) addressed the question raised by Baes (1982) as to 
the capacity of the marine biota for storing carbon in any form. His 
conclusion was that there is so much carbon in seawater as dissolved 
002 in equilibrium with the oceanic carbonate-bicarbonate system that 
errors in estimates of oceanic absorption of C0 2 are most likely to 
involve rates of mixing of surface waters into intermediate or greater 
depths. The issue remains unresolved. If the terrestrial biota appear 
to be a substantial net source of CO 2 for the atmosphere beyond the 
fossil fuel source, the oceans must be absorbing substantially more 
C0 2 than has been measured. 

The discrepancies are emphasized further by consideration of the 
fraction of the CO 2 released that remains in the atmosphere. The 
annual increase in C0 2 in the atmosphere is caused by the accumulation 
of some fraction of the total C0 2 released. Because the fossil fuel 
C0 2 has been thought to be the major, and sometimes the only, source 
of additional CO 2 , a frequent practice has been to express the 
increase in atmospheric C0 2 as a fraction of the fossil fuel release. 
The fraction calculated in this way has the advantage of being based on 
two numbers that are measured with considerable accuracy. The fraction 
has the further advantage of being expected to approach a constant in 
the simplified models commonly used (Bacastow and Keeling 1979, 1981). 

The airborne fraction, calculated solely on the basis of the Mauna 
Loa data and estimates of combustion of fossil fuels was 0.55 for the 
period 1959-1978, according to Bacastow and Keeling (1981). The range 
of airborne fractions consistent with current carbon-cycle models was 
explored by Oeschger and Heimann (1983) . They suggest that a range 



233 

from about 0.4 to 0.7 is possible. If the fraction is less than 0.4, 
there are deficiencies in current descriptions of the carbon cycle. 

When the fossil fuel contribution alone is considered, the 
preindustrial atmospheric 062 concentration is usually thought to 
have been 290-300 ppm. Recognition that there has been a large 
additional release from the biota and soils is consistent with recent 
measurements and estimates of a lower preindustrial concentration (See 
Machta, Section 3.4). If the biotic release were larger during the 
latter part of the last century than now, the airborne fraction would 
have a curious time history. To illustrate, we can calculate the 
airborne fraction for two periods, 1860-1958 and 1959-1980, using the 
estimate of biotic releases based on population (see Table 3.4 above) 
and data from Rotty (1981, 1982) on releases from combustion of fossil 
fuels. 

Time Biotic Fossil Total Atmospheric Airborne 
Period Release Release Release Increase Fraction 



1860-1958 123 76 199 106 0.53 

1959-1980 57 86 143 49 0.34 
Total 180 162 342 155 0.45 



The estimates are made on the assumption, taken arbitrarily, that 
the preindustrial C0 2 concentration in the atmosphere was 265 ppm. 
During the earlier period, 1860-1958, 53% of the total C0 2 thought to 
have been released to the atmosphere appears to have remained there. 
During the latter period, about 34% of the total release seems to have 
accumulated. Over the entire 120 years about 45% of the total has 
remained in the atmosphere according to these estimates. 

A circumstance in which the fraction of atmospheric carbon trans- 
ferred into the oceans or other sinks increases as the concentration of 
C02 in the atmosphere rises is difficult to envision. No persuasive 
explanation is available. Mass balance considerations do not appear to 
support the hypothesis that entrophication of coastal waters, for 
instance, is causing accelerated sedimentation of carbon currently 
(Peterson, 1982) . There do not appear to be mechanisms for sequestering 
sufficient carbon on land. The uncertainty of the basic numbers, 
especially the preindustrial CO2 concentration and the magnitude and 
timing of biotic releases, both used to estimate the airborne fraction, 
re-emphasize that little should be inferred at present from calcula- 
tions such as these. 

There is, nonetheless, ample basis for arguing that the total 
release of carbon into the atmosphere has been and remains larger than 
the release from fossil fuels alone. Such a release means that the 
total accumulation in the atmosphere has been a lower fraction of the 
total release than estimated solely on the basis of combustion of 
fossil fuels. Presumably there has been a greater transfer to the 
oceans than commonly recognized. 



234 

3.3.6 A Projection of Further Releases from Biotic Pools 

If the analyses above are correct, the most important biotic transition 
that affects the global carbon cycle is the destruction of forests. 
The rate of destruction and the rate of release of carbon to the atmo- 
sphere can be anticipated. If the rate is proportional to the increase 
in population as assumed in the intermediate analysis reported above 
from Houghton et al. (1983) , and it is assumed further that population 
continues to grow through the year 2000 and that the rate of growth 
declines thenceforth to zero in the year 2100 , the release from biotic 
sources might be expected to follow the solid line of Figure 3.13. 
Releases would range between the current estimate of approximately 2 Gt 
of C annually to a maximum of between 6 and 7 Gt of C in 2000. If the 
growth in population were to continue beyond 2000, the forests them- 
selves would limit the release by the year 2030 to a maximum of about 
10 Gt of C annually. Forests would then cease to exist as closed 
stands; the impoverished stands would decline progressively in carbon 
content and productivity. Rapid oxidation of 230 Gt of C, roughly the 
current total carbon content of tropical forests according to Olson 
(1982) , would lead to an appreciable increase of atmospheric CO 2 (see 
Machta, Section 3.6) . 

3.3.7 Summary and Conclusions 

The biota and soils of the Earth contain more than three times as much 
carbon as the atmosphere. The most powerful evidence for the importance 
of the biota in affecting the C0 2 content of the atmosphere is the 
annual oscillation in the CO 2 concentration observed at Mauna Loa and 
in virtually every other annual record of atmospheric CO 2 . The extent 
of the biotic influence and the factors that govern it remain uncertain. 
The influence, however, is due primarily to global changes in forest 
biomass . 

Forests have two types of effects on atmospheric CC^, a shorter- 
term effect that is apparent in the oscillation in the concentration of 
CC>2 and a longer-term effect due to changes in the total amount of 
carbon stored in them. The largest change in the mass of carbon in 
forests appears to be a net global reduction due to deforestation to 
support the expansion of agriculture. The biotic release from all 
changes in area, expressed as a net for the world as a whole, is 
probably in the range of 1.8-4.7 Gt of C annually. This conclusion is 
derived from tabulations of data on rates of deforestation and forest 
harvest evaluated with a model. The model provides for the recovery of 
forests following harvest or abandonment of agriculture. Most studies, 
when expressed globally with adjustments to include soils and suc- 
cessional recovery, produce results that fall within the range stated. 

The question remains as to whether the metabolism of forests is 
being affected to change the storage of carbon in otherwise untouched 
stands. Such a change would require an increase in the spread between 
gross photosynthesis and total respiration. The factors that are most 
likely to affect this spread are light, moisture, nutrients, and tern- 



235 



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1860 1880 1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 

YEAR 

FIGURE 3.13 Predictions of the net release of carbon from the biota 
over the next several decades based on the assumption that deforestation 
is proportional to growth in population. The solid line is the net 
release on the assumption that the population continues to grow until 
the year 2000, when growth then declines exponentially to zero; the 
dashed line shows the release if the population continues to grow and 
forests simply disappear early in the next century. For comparison , 
Nordhaus and Yohe (this volume, Chapter 2, Section 2.1) offer a mean 
estimate of fossil fuel C0 2 emissions of about 5 Gt of C in the year 
2000, 10 Gt of C in 2025, 15 Gt of C in 2050, and 20 Gt of C in 2100. 
Deforestation is not a simple function of growth in population; it is 
in part due to economic and industrial circumstances. Nonetheless, for 
this prediction the assumption as made here is adequate to show that 
releases of C0 2 from deforestation could increase well into the next 
century and then decline. 



perature, in addition to the concentration of CC^. Of these, 
temperature would seem to have the greatest potential: a 1C change 
might be expected to produce a 20-30% change in rate of respiration. 
Any effect on total photosynthesis would be largely through an increase 
in the length of the growing season. An increase in the growing season 
with impact on photosynthesis comparable with that of temperature on 
respiration would not be expected from a 1C increase in temperature. 
Interpretations of the apparent increase in amplitude of the oscilla- 



236 

tion of CC>2 concentration at Mauna Loa as due to increased regrowth 
of forests or stimulation of carbon storage on land are premature; the 
amplitude is affected by several factors. The probability seems high 
that a global warming will, at least initially, stimulate respiration 
in existing forests, thereby releasing additional stored carbon to the 
atmosphere? offsetting this trend could be a longer-term expansion of 
the forested zone poleward, with additional storage of carbon in the 
expanded area of forests. 

Recognition that there has been an appreciable biotic release in 
addition to the release of carbon from combustion of fossil fuels has 
important implications both for estimating the seriousness of C0 2 
increase and for potential mitigation of CO 2 -induced changes in 
climate. If there has been an appreciably larger total release than 
generally accepted, then the average fraction of total anthropogenic 
emissions (fossil fuel and biotic) remaining airborne over recent 
decades has a value near 0.4 (Clark et al. in Clark, ed., 1982; 
Woodwell et al., 1983a) . Equally important, a total release from the 
biota and soils of approximately 180 Gt of C over the past 120 years 
would be consistent with an atmospheric CO 2 concentration in the 
mid-nineteenth century in the lower part of the 250-290-ppm range. 
Independent evidence is growing for the lower part of the range. 
Finally, the potential of the biota, especially forests, to release or 
store carbon is large enough to affect the C0 2 content of the 
atmosphere significantly year by year. If the surge in use of fossil 
fuels continues, the relative importance of the biotic contribution 
will diminish. If fossil use increases at a moderate rate, management 
of the biotic pools of carbon can affect the time that any given 
atmospheric CO 2 concentration is reached by several decades. Better 
prediction, even control, of future C0 2 concentrations are possible 
but will require substantially strengthened research and understanding 
of the carbon cycle. 



References 

Adams, J. A. S., M. S. M. Mantovani, and L. L. Lundell (1977). Wood 
versus fossil fuel as a source of excess carbon dioxide in the 
atmosphere: a preliminary report. Science 196; 54-56. 

Ajtay, G. L., P. Ketner, and P. Duvigneaud (1979). Terrestrial primary 
production and phytomass. In The Global Carbon Cycle, B. Bolin et 
al., eds. SCOPE Report 13. Wiley, New York, pp. 129-181. 

Bacastow, R., and C. D. Keeling (1973). Atmospheric carbon dioxide and 
radiocarbon in the natural carbon cycle. II: Changes from A.D. 1700 
to 2070 as deduced from a geochemical model. In Carbon in the 
Biosphere f G. M. Woodwell and E. V. Pecan, eds. USAEC Symposium 
Series No. 30., U.S. Dept. of Commerce. NTIS, Springfield, Va., pp. 
86-136. 

Bacastow, R. B., and C. D. Keeling (1979). Models to predict future 
atmospheric CO 2 concentrations. In Workshop on the Global 
Effects of Carbon Dioxide from Fossil Fuels, W. P. Elliott and L. 



237 

Machta, eds. U.S. Dept. of Energy, CONF-7 70385, NTIS, Springfield, 
Va., pp. 72-90. 

Bacastow, R., and C. D. Keeling (1981) Atmospheric carbon dioxide 
concentration and the observed airborne fraction. In Carbon Cycle 
Modelling, B. Bolin, ed. SCOPE Report 16. Wiley, New York, pp. 
103-112. 

Bacastow, R. B., C. D. Keeling, T. P. Whorf, and C. S. Wong (1981a) . 
Seasonal amplitude in atmospheric CC>2 concentration at Canadian 
weather station P, 1970-1980. Papers presented at WMO/ICSU/UNEP 
Scientific Conference on Analysis and Interpretation of Atmospheric 
C02 Data, Bern, 1981. World Climate Programme. 

Bacastow, R. B., C. D. Keeling, and T. P. Whorf (1981b) . Seasonal 
amplitude in atmospheric CO 2 concentration at Mauna Loa, Hawaii, 
1959-1980. Papers presented at WMO/ICSU/UNEP Scientific Conference 
on Analysis and Interpretation of Atmospheric C0 2 Data, Bern, 
1981. World Climate Programme. 

Baes, C. F. (1982) . Effects of ocean chemistry and biology on 

atmospheric carbon dioxide. In Carbon Dioxide Review: 1982, W. C. 
Clark, ed. Oxford U. Press, New York, pp. 187-204 

Billings, W. D., J. O. Luken, D. A. Mortensen, and K. M. Peterson 
(1982) . Arctic tundra: A source sink for atmospheric carbon 
dioxide in a changing environment? Oecologia 53; 7-11. 

Bohn, H. L. (1976) . Estimate of organic carbon in world soils. Soil. 
Sci. Soc. Am. J. 40; 468-470 . 

Bolin, B. (1977) . Changes of land biota and their importance for the 
carbon cycle. Science 196; 613-615. 

Broecker, W. S. (1974). Chemical Oceanography. Harcourt Brace 
Jovanovich, New York. 

Broecker, W. S., T. Takahashi, H. J. Simpson, T.-H. Peng (1979). Fate 
of fossil fuel carbon dioxide and the global carbon budget. Science 
2 06; 4 09-418. 

Brown, C. W., and C. D. Keeling (1965). The concentration of 
atmospheric carbon dioxide in Antarctica. J. Geophys. Res. 
J70; 6077-6085. 

Brown, S., and A. E. Lugo (1981). The role of the terrestrial biota in 
the global C02 cycle. Proceedings of a Symposium: A Review of 
the Carbon Dioxide Problem. Am. Chem. Soc., Div. Petrol. Chem. 
26.; 1019-1025. 

Buringh, P. (1983). Organic carbon soils of the world. In The Role of 
Terrestrial Vegetation in the Global Carbon Cycle; Measurement by 
Remote Sensing, G. M. Woodwell, ed. SCOPE. Wiley, New York (in 
press) . 

Clark, W. C., ed. (1982). Carbon Dioxide Review; 1982. Oxford U. 
Press, New York. 

Detwiler, R. P., C. A. S. Hall, and P. Bogdonoff (1981). Simulating 
the impact of tropical land use changes on the exchange of carbon 
between vegetation and the atmosphere. In Global Dynamics of 
Atmospheric Carbon, S. Brown, ed. Proceedings of a symposium held 
at the annual meeting of the Ecological Society of America under the 
auspices of the American Institute of Biological Sciences, 
Bloomington, Indiana, 1981. U.S. Dept. of Energy, pp. 141-159 



238 

Eckhardt f P. E. , L. Heerfordt, H. M. Jorgensen, and P. Vaag (1982). 

Photo-synthetic production in Greenland as related to climate f plant 

cover , and grazing pressure. Photosynthetics 16; 71-100* 
Giese, A. C. (1968). Cell Physiology . 3rd ed. Saunders, 

Philadelphia, Pa. 
Gifford, R. M. (1979). C0 2 and plant growth under water and light 

stress: implications for balancing the global carbon budget. 

Search 10:316-318. 
Hall, C. A. S., C. A. Ekdahl, and D. E. Wartenberg (1975). A fifteen 

year record of biotic metabolism in the northern hemisphere. Nature 

255:136-138. 

Hampicke r U. (1979) . Sources and sinks of carbon dioxide in ter- 
restrial ecosystems. Environ. Internat. 2:301-316. 
Houghton, R. A., J. E. Bobbie, J. M. Melillo, B. Moore, B. J. Peterson, 

G. R. Shaver, and G. M. Woodwell (1983). Changes in the carbon 

content of terrestrial biota and soils between 1860 and 1980: A net 

release of C02 to the atmosphere. Ecolog. Mono-y^ (in press). 
Keeling, C. D., J. A. Adams, Jr., C. A. Ekdahl, and P. R. Guenther 

(1976a) . Atmospheric carbon dioxide variation at the South Pole. 

Tellus 28:552-564. 
Keeling, C. D., R. B. Bacastow, A. E. Bainbridge, C. A. Ekdahl, Jr., 

P. R. Guenther, L. S. Waterman, and J. F. S. Chin (1976b) . 

Atmospheric carbon dioxide variations at Mauna Loa Observatory, 

Hawaii. Tellus 28; 538-551. 
Kelley, J. J. (1969) . An analysis of carbon dioxide in the arctic 

atmosphere near Barrow, Alaska, 1961 to 1967. University of 

Washington, Dept. of Atmospheric Sciences, Scientific Report, Naval 

Research Contract No. 614-67-A-0103-0007. NR. 307-252, Seattle, 

Washington. 
Kramer, P. J. (1981). Carbon dioxide concentration, photosynthesis and 

dry matter production. BioScience 31:29-33. 
Larcher, W. (1980). Physiological Plant Ecology. Springer-Verlag, 

Berlin. 
Moore, B., R. D. Boone, J. E. Hobbie, R. A. Houghton, J. M. Melillo, 

B. J. Peterson, G. R. Shaver, C. J. Vorosmarty, and G. M. Woodwell 

(1981). A simple model for analysis of the role of terrestrial 

ecosystems in the global carbon budget. In Carbon Cycle Modelling, 

B. Bolin, ed. SCOPE Report 16. Wiley, New York. 
Myers, N. (1980). Conversion of Tropical Moist Forests. Prepared for 

Committee on Research Priorities in Tropical Biology, National 

Research Council, National Academy of Sciences, Washington, D.C. 
Oeschger, H., and M. Heimann (1983). Uncertainties of predictions of 

future atmospheric C02 concentrations. J. Geophys. Res. 

88.: 1258-1262. 
Olson, J. S. (1982). Earth's vegetation and atmospheric carbon 

dioxide. In Carbon Dioxide Review: 1982, W. C. Clark, ed. Oxford 

U. Press, New York, pp. 388-398. 
Pear man, G. I., and P. Hyson (1981). The annual variation of 

atmospheric C0 2 concentration observed in the Northern 

Hemisphere. J. Geophys. Res. 86; 9836-9843. 



239 

Persson, R. (1974). Review of the world's forest resources in the 

early 1970s. Dept. of Forest Survey Res. Notes No. 17. Royal 

College of Porestry r Stockholm. 265 pp. 
Persson r R. (1977) . Scope and approach to world forest resource 

appraisals. Dept. of Forestry Survey Res. Notes No. 23. Royal 

College of Forestry, Stockholm. 
Peterson, B. J. (1982) . In Carbon Dioxide Review; 1982, W. C. Clark, 

ed. Oxford U. Press, New York, pp. 205-206. 
Post, W. M., W. R. Emanuel, P. J. Zinke, and A. G. Stangenberger 

(1982). Soil carbon pools and world life zones. Nature 298 : 156-159 . 
Rebello, A., and K. Wagener (1976). Evaluation of 1- *C and 1J C data 

on atmospheric CO 2 on the basis of a diffusion model for oceanic 

mixing. In Environmental Biogeochemistry, Vol. 1; Carbon, 

Nitrogen, Phosphorus, Sulfur, and Selenium, J. O. Nriagu, ed. Ann 

Arbor Science Publishers, Ann Arbor, Mich., pp. 13-23. 
Reichle, D. E., B. E. Dinger, N. T. Edwards, W. F. Harris, and P. 

Sollins (1973) . Carbon flow and storage in a forest ecosystem. In 

Carbon and the Biosphere , G. M. Woodwell and E. V. Pecan, eds. U.S. 

Atomic Energy Commission, pp. 345-365. 
Revelle, R., and W. Munk (1977). The carbon dioxide cycle and the 

biosphere. Energy and Climate. National Research Council, 

Geophysics Study Committee, National Academy Press, Washington, 

D.C., pp. 140-158. 
Richards, J. F., J. S. Olson, and R. M. Rotty (1983). Development of a 

data base for carbon dioxide releases resulting from conversion of 

land to agricultural uses. ORAU/IEA-82-10 (M) . Institute for Energy 

Analysis, Oak Ridge, Tenn. 
Rotty, R. M. (1981) . Data for global CO 2 production from fossil 

fuels and cement. In Carbon Cycle Modelling, B. Bolin, ed. SCOPE 

Report 16. Wiley, New York, pp. 121-125. 
Rotty, R. M. (1982) . Distribution and changes in industrial carbon 

dioxide production. J. Geophys. Res. 88; 1301-1308. 
Schlesinger, W. H. (1977). Carbon balance in terrestrial detritus. 

Ann. Rev. Ecol. Syst. 8; 51-81. 
Schlesinger, W. H. (1983). The world carbon pool in soil organic 

matter: a source of atmospheric CO 2 . In The Role of Terrestrial 

Vegetation in the Global Carbon Cycle; Measurement by Remote 

Sensing. SCOPE. Wiley, New York. 
Seiler, W., and P. J. Crutzen (1980). Estimates of gross and net 

fluxes of carbon between the biosphere and the atmosphere from 

biomass burning. Climatic Change 2:207-247. 
Siegenthaler, U., and H. Oeschger (1978). Predicting future 

atmospheric carbon dioxide levels. Science 199:388-394. 
Smith, S. V. (1981). Marine macrophytes as a global carbon sink. 

Science 211:838-840. 
Strain, B. R. , ed. (1978). Report of the workshop on anticipated plant 

responses to the global carbon dioxide enrichment, held August 4-5, 

1977. Dept. of Botany, Duke U., Durham, N.C. 91 pp. 
Stuiver, M. (1978) . Atmospheric carbon dioxide and carbon reservoir 

changes. Science 199: 253-270 . 



240 

Tans, P. P. (1978) . Carbon 13 and Carbon 14 in trees and the 

atmospheric C0 2 increase. Thesis. Rijsuniversiteit te Groningen, 

The Netherlands. 
Wagener, K. (1978). Total anthropogenic CO 2 production during the 

period 1800-1935 from carbon-13 measurements in tree rings. Radiat. 

Environ. Biophys. 15; 101-111 . 
Walsh, J. J. r G. T. Rowe, R. L. Iverson, and C. P. McRoy (1981). 

Biological export of shelf carbon is a sink of the global C0 2 

cycle. Nature 291; 196-201. 
Whittaker, R. H., F. H.Bovmann, G. B. Likens, and T. G. Siccama 

(1974) * The Hubbard Brook ecosystem study: forest biomass and 

production. Ecolog. Manag. 44; 233-254 . 
Whittaker, R. H. , and G. E. Likens (1973). Carbon in the biota. 

In Carbon and the Biosphere, G. M. Woodwell and E. V. Pecan, eds. 

USAEC Symposium Series No. 30. NTIS, Springfield, Va., pp. 281-302. 
Whittaker, R. H., and G. M. Woodwell (1969). Structure, production and 

diversity of the oak-pine forest at Brookhaven, New York. J. Ecol. 

$2*. 155-174. 
Woledge, J., and W. D. Dennis (1982). The effect of temperature on 

photosynthesis of ryegrass and whole clover leaves. Ann. Bot. 

10:25-35. 
Wong, C. S. (1978). Atmospheric input of carbon dioxide from burning 

wood. Science 200; 197-199. 
Wong, C. S. (1979) . Elevated atmospheric partial pressure of C0 2 and 

plant growth. Oecologia 44;68-74. 
Woodwell, G. M., ed. (1980). Measurement of changes in terrestrial 

carbon using remote sensing. U.S. Dept. of Energy CONF-7905176, 

UC-11. Available from NTIS, Springfield, Va. 
Woodwell, G. M., ed. (in press). The Role of Terrestrial Vegetation in 

the Global Carbon Cycle; Measurement by Remote Sensing. SCOPE 

Report 23. Wiley, New York. 
Woodwell, G. M., and D. B. Botkin (1970). Metabolism of terrestrial 

ecosystems by gas exchange techniques; the Brookhaven approach. In 

Ecological Studies y D. E. Reichle, ed. Analysis and Synthesis, 

Volume 1. Springer-Verlag, Berlin, pp. 73-85. 
Woodwell, G. M., and R. A. Houghton (1977). Biotic influences on the 

world carbon budget. In Global Chemical Cycles and Their Alteration 

by Man, W. Stumm, ed. Report of the Dahlem Workshop, November 

15-19, 1976. Dahlem Konferenzen, Berlin, pp. 61-72. 
Woodwell, G. M., and R. H. Whittaker (1968). Primary production in 

terrestrial ecosystems. Am. Zool. 8;19-30. 
Woodwell, G. M., R. A. Houghton, and N. R. Tempel (1973a) . Atmospheric 

C0 2 at Brookhaven, Long Island, New York; patterns of variation 

up to 125 meters. J. Geophy. Res. 78; 932-940. 
Woodwell, G. M. , P. H. Rich, and C. A. S. Hall (1973b) . Carbon in 

estuaries. In Carbon in the Biosphere, G. M. Woodwell and E. V. 

Pecan, eds. Proceedings of the Twenty-fourth Brookhaven Symposium 

in Biology, Upton, New York. USAEC, Office of Information Sources. 

NTIS, Springfield, Va., pp. 221-240. 



241 

Woodwell, G. M., R. H. Whittaker, W. A. Reiners, G. E. Likens/ C. C. 
Delwiche, and D. B. Botkin (1978) . The biota and the world carbon 
budget. Science 199; 141-14 6. 

Woodwell, G. M. , R. A. Hough ton, C* A. S. Hall, D. . Whitney, R. A. 
Moll, and D. W. Juers (1979). The Flax Pond ecosystem study: the 
annual metabolism and nutrient budgets of a salt marsh. In 
Ecological Processes in Coastal Environments, R. L. Jeffries and A. 
J. Davy, eds. The First European Ecological Symposium and the 
Nineteenth Symposium of the British Ecological Society, Norwich, 
September 1977. Blackwell Scientific Publications, Boston, Mass., 
pp. 491-511. 

Woodwell, G. M., J. E. Hobbie, R. A. Houghton, J. M. Melillo, B. Moore, 
C. A. Palm, B. J. Peterson, and G. R. Shaver (1982) . Report of the 
Woods Hole Conference on the Biotic Contributions to the Global 
Carbon Cycle at the Ecosystems Center. Ecosystems Center, Marine 
Biological Laboratory, Woods Hole, Mass., March 1982. 

Woodwell, G. M., J. E. Hobbie, R. A. Houghton, J. M. Melillo, B. Moore, 
B. J. Peterson, and G. R. Shaver (1983a) . The contributions of 
global deforestation to atmospheric carbon dioxide. Photocopies 
available from the Ecosystem Center, Marine Biological Laboratory, 
Woods Hole, Mass. 

Woodwell, G. M. , J. E. Hobbie, R. A. Houghton, J. M. Melillo, B. J. 
Peterson, G. R. Shaver, T. A. Stone, B. Moore, and A. B. Paru 
(1983b). Deforestation measured by LANDS AT: steps toward a 
method. DOE/EV/10468-1. NTIS, Springfield, Va. 

Zelitch, I. (1971). Photosynthesis, Photorespiration, and Plant 
Productivity. Academic, New York. 



242 

3.4 THE ATMOSPHERE 
Lester Machta 

3.4.1 Introduction 

It is the growing concentration of CO 2 in the atmospheric reservoir 
that has attracted most attention to the O> 2 issue. The pre- 
Industrial Revolution (e.g./ 1850) concentration probably lay in the 
range 250 to 295 ppmv (parts per million by volume or mole fraction) . 
Measurements by chemical analysis (Callendar, 1958; Keeling/ 1978) and 
extrapolations backward based only on records of fossil fuel emissions 
suggest a late-nineteenth-century concentration at the upper end. 
Measurements from ice cores (Neftel et al., 1982; Oeschger, 1983) and 
reconstructed ocean measurements (see Brewer/ Section 3.2) suggest 
preindustrial concentrations at the lower end. A WMO-sponsored Meeting 
of Experts in June 1983 concluded the most likely mid-nineteenth-century 
concentration was between 260 and 280 ppm/ based on consideration of 
all the various estimates including carbon isotope data in tree rings 
(the meeting report will be issued at a later date) . Concentrations 
significantly less than 290 ppm imply the existence of a large nonfossil 
fuel source of C(>2 and are thus consistent with a large early input 
from disturbances of the biosphere. By 1980 the atmospheric C(>2 
concentration had risen to about 340 ppmv. 

The behavior of C02 in air is simpler and better understood than 
in the other two major reservoirs the land biosphere and the oceans. 
C02 is conservative in air/ that is, it is not subject to chemical 
transformation at least up to an altitude of about 60 km. It moves 
with the other inert air molecules with which it is embedded. Most/ if 
not all/ of the known variations of CO 2 in time and space in the air 
appear to follow known meteorological principles. Since interest often 
focuses on time scales of years to decades/ as a first approximation/ 
it is usually acceptable to treat the whole atmosphere as a single 
well-mixed box and apply first-order kinetics to the C0 2 transfer to 
other reservoirs. 

The growth of C0 2 in the air can be demonstrated at almost any 
location on Earth over a period of several years. Modern-day measure- 
ments were begun by C. D. Keeling of Scripps Institution of Oceanography 
during the 1957-1958 International Geophysical Year. Stations were 
established at the South Pole and at 11,150 feet aside Mauna Loa in 
Hawaii. The latter/ the better record/ is reproduced in Figure 3.14. 
Since the pioneering measurements of Keeling/ other stations operated 
by many countries and organizations have been established. A map of 
the location of stations as of July 1982 as supplied by the World 
Meteorological Organization (WMO) appears as Figure 3.15. At most of 
the stations air is collected in containers for subsequent analysis in 
central laboratories. With few exceptions/ both on-station and lab- 
oratory analyses are performed by nondispersive infrared analysis 
comparing the ambient samples with standard gases. Since the response 
of the analyzer depends on the carrier gas/ it is now agreed that the 
carrier gas of the standard should duplicate air as closely as possible. 



243 

e.g.f nitrogen, oxygen, and argon instead of the former nitrogen gas. 
A transition to standards of C0 2 in air (or simulated air) now in 
progress around the world should proceed as quickly as possible and be 
consistent with ensuring long-term integrity of the standards. 
Carrier-gas corrections based on C0 2 -in-N2 standards are required 
to bring concentrations closer to their true values. 



3.4.2 Changes in Atmospheric CO? Growth Rate with Time and Space 

There appear to be two kinds of changes in the year-to-year growth rate 
at Mauna Loa (Figure 3.14) : a shorter- and longer-term variation. 



3.4.2.1 Shorter-Term Variation and Its Possible Cause 

Here we follow the analysis of Machta et al. (1977); the same result is 
arrived at by the analysis of Bacastow (1976) and Newell and Weare 
(1977) , although a different interpretation is offered by them. The 
monthly mean concentrations at a given station exhibit a seasonal 
oscillation and a long-term trend, both of which can be removed mathe- 
matically. The resulting monthly values of two stations, in this case 




1958 1960 1965 1970 1975 19801981 

(.66) .94 .70 .90 .50 (.69) (.58) .67 .6 .71 .94 1.90 1.33 .87 1.222.22 .57 .60 1.06 1.57 1.571.351.81 

ANNUAL CHANGE (ppmv/yr) 

FIGURE 3.14 Mean monthly concentrations of atmospheric CO 2 at Mauna Loa. 



244 




245 







RESIDUALS FROM FIT TO A + A, ebt 






o I 






i i i i i i 1 i 1 1 I 1 1 i 1 I 1 




400 







300 







200 


- --. 


Annual 




- 


Consumption 


100 


- 


of Fossil Fuel 






Carbon Dioxide 







(x10 12 gC0 2 ) 


-100 







-200 


" _ t 




-300 







1 


. 






* t * 


Mauna Loa 




* ' ^t ***'** . * */ v / / 


(ppm) 




\. m \\ ; f ~^-^ H ^* 9 .* *t - 




-1 


- ^ * ^ 






? 






i * i i i < El Nino 




1 


. 






* . "" fc - 












, * w ^-**- f ^ i- '*. s " *' *^' t l * % 


South Po 
(ppm) 


e 


. ' l -^--- --"^ '"-*vv^: ^r; y^ 




-1 


j' 
. " 






i i i it i i.i i i i"i i i i i t i 






1958 1960 1962 1964 1966 1968 1970 1972 1974197 






YEAR 



FIGURE 3.16 Time history of residuals of monthly carbon dioxide 
concentrations after removing the long-term upward trend and seasonal 
variability at Mauna Loa and the South Pole (lower section) . The 
horizontal lines in the upper section represent the residuals of the 
annual consumption (actually production) of fossil fuels after removing 
the long-term upward trend. The periods of El Nino are also shown in 
the lower section. The horizontal lines in the lower section among the 
circles and crosses are the annual average residuals. 



Mauna Loa (small circles) and the South Pole (crosses) , appear in 
Figure 3.16. This figure shows a pattern of short-term variation with 
fluctuations reversing themselves every 2 to 6 years. The horizontal 
bars among the circles and crosses represent the annual values of the 
12 monthly averages. The horizontal bars in the upper part of Figure 
3.16 represent the departures from the best exponential fit of the 
annual fossil fue x l emissions of CO 2 from the mean value for the 17 
years. 

Variation in annual emissions might be the reason for the year-to- 
year fluctuation in Figure 3.16 of the atmospheric CO 2 content; 
however, the correlation coefficient between concurrent anomalies of 
emissions and atmospheric content is only 0.42, not statistically 
significant. Allowing for a lag of up to 6 years between emissions and 
atmospheric CO 2 content simply reduces the correlation coefficient 



246 

below 0.42. While year-to-year fluctuations in fossil fuel combustion 
contribute to variability in atmospheric CC>2 concentrations! other 
factors appear to dominate the variability. 

There seems to be a good correspondence between increasing anomalies, 
the open circles f and the periodic variations in the southern hemisphere 
oceans and atmosphere known as 1 Nino events, shown as the horizontal 
bars between the record at the two stations. A similar relationship can 
be found between the anomalies in atmospheric C0 2 content and tempera- 
ture in the tropical eastern Pacific Ocean and with the Southern Oscil- 
lation. An analysis using a two-dimensional transport model (vertical 
and north-south directions) suggests that the lag of changing concentra- 
tions among stations (Mauna Loa, the South Pole and Australia) fits a 
ground- or sea-level source or sink of CX>2 (i.e., a forcing function) 
near 5 to 10 S, the region of the El Nino. But the cause of the 
forcing function in the tropical Pacific is less clear* The warmer 
sea-surface temperatures associated with the El Nino could produce a 
higher-than-normal partial pressure of CO2, enhancing the tropical 
oceanic source; the warmer temperatures also reflect lesser upwelling, 
which reduced the oceanic biological activity, which, in turn, can 
affect the air-sea C0 2 difference in the same sense as the warmer 
water (e.g., a smaller biosphere will take up less atmospheric C0 2 ) ; 
and finally, the changing wind speeds related to the Southern 
Oscillation can also alter the rate of air-sea exchange of CO 2 . 

Thus, the above analysis, while leaving questions about the cause of 
the forcing function unanswered, does indicate that the shorter-term (2 
to 6 years) variations in atmospheric 002 concentrations are 
empirically correlated to some phenomena in the eastern tropical 
Pacific Ocean (El Nino, Southern Oscillation, biota change) . 



3.4.2.2 Longer-Term Variations 

An inspection of year-to-year increases in concentration at Mauna Loa 
reveals that they are generally becoming larger with time. Figure 3.14 
shows this trend in the values of annual changes in ppmv/yr. Through 
1968 the annual increase was below 1 ppmv, while in recent years it has 
often been nearer to 1.5 ppmv. The emissions of man-made C0 2 (mainly 
fossil fuel combustion plus cement manufacture and flaring of natural 
gas*) are also becoming larger with time. Elliott (1983) estimates an 
overall annual growth rate of emissions from industrial activity of 
about 3.5% over the past 120 years, with wide variation due to economic 
fluctuations, and Nordhaus and Yohe (this volume, Chapter 2, Section 
2.1) predict that emissions from fossil fuels will most likely grow at 
a rate of about 1% or 2% per year over the next hundred years. But 
there are other likely sources for atmospheric C0 2 ; in particular the 
CO 2 produced as a result of deforestation. Elliott and Machta (1981) 



*Hereafter, the term "fossil fuel CO 2 n is understood to include 
the other two minor contributions as well. 



247 

have sought to determine whether the Mauna Loa and South Pole records 
of C02 increase (the average of the two is taken as the average for 
the Earth in this analysis) are better fitted by the increasing fossil 
fuel combustion source of 002 or whether the addition of another 
significant source, such as from deforestation, results in a better fit 
to the measurements. In principle, if there were an accurate model of 
the carbon cycle, one might enter alternate amounts of C0 2 into it, 
and the best fit to the observed data would be the best size of the 
source to fit observed concentrations. 

Elliott and Machta (1981) have tried to avoid the issue of defining 
the carbon cycle. They assume that each year roughly the same fraction 
of that year's CO 2 emissions remains airborne. This airborne fraction 
is determined from the ratio of the C0 2 increase in the atmosphere 
(as found from the average annual increases at Mauna Loa and the South 
Pole) to the amount added to the air from all sources that one wishes 
to assume. The interval of this analysis is from 1958 to 1981. The 
result indicates that the observed increases in atmospheric CO 2 are 
best fitted by only a fossil fuel source, without any additional 
constant or random CO 2 source. In fact, the analysis suggests that 
there could be a small loss of CO 2 from the atmosphere, possibly 
through C0 2 fertilization of photosynthesis because of the elevated 
atmospheric CO 2 concentration. There is one caveat, however. This 
analysis could not distinguish between fossil fuel and nonfossil fuel 
emissions, such as deforestation, that are increasing at the same rate. 
It does, however, provide estimates of how closely the growth of the 
nonfossil source had to match the fossil fuel growth to be undetectable 
For sources averaging more than 2 Gt of C per year, the growth rate 
would have to have been quite close to the fossil fuel growth rate to 
be undetected. 



3.4.2.3 Change in Annual Cycle 

The annual cycle shown in Figure 3.14 for Mauna Loa CO 2 concentrations 
is almost certainly the result of the warm season uptake of CO 2 during 
land biosphere photosynthesis (see Woodwell, Section 3.3). There could 
be a contribution or diminution from the seasonality of fossil fuel com- 
bustion or the air-sea exchange of C0 2 . The seasonality of the latter 
processes are believed to be small in determining the annual cycle of 
atmospheric CO 2 concentrations. 

The long, high-quality record at Mauna Loa has been analyzed to 
determine whether the amplitude of the annual cycle is changing with 
time. The result is shown in Figure 3.17. The increase of amplitude 
suggested by a best-fit line, the dashed line, in large part depends on 
an increase that occurs only during the past 6 years. The finding of 
increasing amplitude at Mauna Loa is supported by analysis of a shorter 
and less convincing C0 2 record at the Canadian ship Papa located in 
the Gulf of Alaska, according to Keeling (1983) . The most plausible 
explanation of the increasing amplitude is increased biological activ- 
ity, such as, but not necessarily, a larger temperate and high-latitude 
biosphere (e.g., larger forests) (c.f., Woodwell, Section 3.3). 



248 



120 



olOO 
a: 

UJ 

-90 



80 
110 
100 



Relative Amplitude 
of the Seasonal Cycle 



STN 'P' 50 N 




SLOPE* 
0.77 %/YR 



UJ 



90 



80 





\ 



SLOPED 
0.66% /YR 



MAUNA LOA (19 N) 



1958 '60 '62 '64 '66 '68 '70 '72 '74 '76 '78 '80 



(b) 



YEAR 



FIGURE 3.17 Seasonal amplitude in atmospheric CO 2 concentration at 
(a) Weather Ship P at 50.0 N and (b) Hauna Loa Observatory at 19.5 
N. Dots connected by solid lines represent an estimation of the 
amplitude for individual years as determined by a best fit of a 
four-harmonic seasonal cycle as described by Bacastow et al. (1981). 
The dashed straight line is a least-squares fit of a linearly 
increasing amplitude over the entire period of record. (Source: 
Keeling, 1982.) 



3.4.2.4 Spatial Distribution 

Keeling (1982) has provided north-south profiles of ground-level air 
concentrations of CC>2 relative to that at the South Pole (see Brewer, 
Section 3.2). Profiles for 3 years, all adjusted to a common value at 
the South Pole are shown in Figure 3.9 in Section 3.2. It is evident 
that the secular growth in the northern hemisphere exceeds that in the 
southern hemisphere, as might be expected from the location of fossil 
fuel C0 2 sources mainly in the northern hemisphere. The concentration 
in each of the 3 years is also higher in the northern than southern 
hemisphere. Near the equator there is a secondary peak, which might be 
due to either the release of C0 2 from the tropical oceans or defor- 
estation in the tropics. Keeling has estimated the transfer from the 
sea to air and compared this with the equatorial peak in Figure 3.18. 
His conclusion is that C0 2 from tropical deforestation during 
1962-1980 is unlikely to exceed 1 or 2 Gt of C per year or no more than 



249 



3.0 



a 

3 2.0 



LU 

a. 
c/D 
O 

1 






1 i I Ij 



1980 




1968 



1962 



[] J J 



I 1 I 



J 1 I 1 I t I I l 



90 S 



40 S 20 S 



20 N 40 N 



90 N 



LATITUDE (deg) 



FIGURE 3.18 North-south profile of ground-level air concentration 
relative to that at the South Pole for 3 years. 



40% of the current C0 2 released from fossil fuel combustion and 
likely much less. 



3.4.2.5 Isotopic Content of Atmospheric C(>2 

Isotopic ( 1: *C) analyses of atmospheric CO2 samples have been 
undertaken in a systematic fashion in only the past few years so that 
conclusions derived from these data must still be viewed with caution. 
Keeling (1983) contends that the seasonal cycle in 13 C measurements 
are consistent with land plants being the primary source of the annual 
cycle of C0 2 concentration for northern hemisphere stations. At the 
South Pole the isotopic data suggest an oceanic source for the cause of 
the much smaller annual cycle. Tentative results from Keeling from 
Fanning Island in the tropical Pacific Ocean and during oceanographic 
cruises in the tropics (the First Global Atmospheric Research Program 
Global Experiment expedition) support the contention that the peak C0 2 
concentration in the equatorial region in Figure 3.18 is the result of 
air-sea exchange and not due to deforestation in the tropics. Leavitt 



250 

and Long (1983) , while allowing for other interpretations, report that 
the shape of the best-fit reconstruction of 50 yr of 1 ^ C /12 C measure- 
ments from tree rings suggests that the biosphere has acted as a C0 2 
source to about 1965 but has become a sink afterward. 



3.4.3 Conclusions 

Atmospheric C0 2 data provide information far beyond the single obser- 
vation that atmospheric CO 2 is increasing. Through careful measure- 
ments, one is able to derive valuable information from the temporal and 
spatial variability. The pattern of results is highly suggestive of a 
minimal contribution of nonfossil fuel sources of C0 2 . Globally, 
during the past 20 years, most of the variations are more readily 
accounted for by the growing fossil fuel source alone than from any 
significant additional source from, say, deforestation. Both the 
limited quantity of data and the possibility of alternate explanations 
prevent any definitive statement today that excludes nonfossil fuel 
sources. 

The need to continue quality observations cannot be overemphasized. 
The spatial gradients of atmospheric CO 2 are so small that the total 
minimum to maximum concentration at a clean air location for the entire 
globe is no more than about 1% of the mean global concentration. Data 
collected with very high precision are needed to detect such small 
gradients. 

Historical and geologic data from past records such as from ice cores 
have proven to be very valuable, and an expanded effort to confirm the 
previous findings (about 200 ppmv found 18,000 years ago) should be 
undertaken. The isotopic studies in tree rings and from current air 
samples offer potential to elucidate further the carbon cycle and the 
contribution of the nonfossil fuel CO 2 . For example, following the 
fate with time of the nuclear weapons test 14 C0 2 in the atmosphere 
can continue to provide new information on the atmospheric residence 
time of the fossil fuel C0 2 . 



References 

Bacastow, R. B. (1976) . Modulation of atmospheric carbon dioxide by 
the Southern Oscillation. Nature 261; 116. 

Bacastow, R. B., C. D. Keeling, and T. P. whorf (1981). Seasonal 
amplitude in atmospheric C0 2 concentration at Mauna Loa, Hawaii 
1959-1980. In papers presented at the WMO/ICSU/UNEP Scientific 
Conference on Analysis and Interpretation of Atmospheric C0 2 
Data. World Meteorological Organization, Geneva, Switzerland, pp. 
169-176. 

Callendar, G. S. (1958). On the amount of carbon dioxide in the 

atmosphere . Tellus 10: 243-248 . 
Elliott, W. P. (1983). A note on the historical industrial production 

of carbon dioxide. Climate Change 5:141-144. 



251 

Elliott, W. P., and L. Machta (1981). In papers presented at the 

WMO/ICSU/UNEP Scientific Conference on Analysis and Interpretation 

of Atmospheric CO 2 Data. World Meteorological Organization, 

Geneva, Switzerland, p. 191. 
Keeling, C. D. (1978) . Atmospheric carbon dioxide in the 19th 

century. Science 202; 1109. 
Keeling, C. D. (1983) . The global carbon cycle: what we know and 

could know from atmospheric, biospheric, and oceanic observations. 

In Proceedings, CO? Research Conference: Carbon Dioxide, Science, 

and Consensus, Berkeley Springs, West Virginia. CONF-820970. NTIS, 

Springfield, Va. 22161. 
Leavitt, S. W., and A. Long (1983). An atmospheric 

reconstruction generated through removal of climate effects from 

tree-ring 13c/12 c measurements. Tellus 35B; 92-102. 
Machta, L., K. Hanson, and C. D. Keeling (1977). Atmospheric carbon 

dioxide and some interpretations. In The Fate of Fossil Fuel CO? 

in the Oceans, N. R. Andersen and A. Malahoff , eds. Plenum, New 

York, pp. 131-144. 
Neftel, A., H. Oeschger, J. Schwander, B. Stauffer, and R. Zumbrunn 

(1982) . New measurements on ice core samples to determine the CO 2 

content of the atmosphere during the last 40,000 years. Nature 

^95:220-223. 
Newell, R. E., and B. C. Weare (1977). A relation between atmospheric 

carbon dioxide and Pacific sea-surface temperature. Geophys. Res. 

Lett. 4;l-2. 
Oeschger, H., and M. Heimann (1983). Uncertainties of predictions of 

future atmospheric C(>2 concentrations, J. Geophys. Res. 88:1258. 



252 

3.5 METHANE HYDRATES IN CONTINENTAL SLOPE SEDIMENTS AND INCREASING 
ATMOSPHERIC CARBON DIOXIDE 
Roger R. Revelle 

3.5.1 Methane in the Atmosphere 

About 4.8 Gt of methane (CH 4 ) are present in the Earth's atmosphere, 
corresponding to 1.7 ppm by volume (see Machta, this volume. Chapter 4, 
Section 4.4) . Methane is a strong absorber of infrared radiation in 
the part of the atmospheric "window" centered around a wavelength of 
7.66ym. According to Lacis et al. (1981), a doubling of the 
atmospheric methane concentration would cause an increase in global 
average surface temperature of 0.41C. Chamberlain et al. (1982) 
estimate a larger value, 0.95C for methane doubling and report lower 
and higher results by other groups as well. These calculations allow 
for positive feedbacks resulting from the increase in absolute humidity 
with rising temperatures and the consequent higher infrared absorption 
by water vapor, decreases of planetary albedo due to melting of snow 
and ice, and assumed cloud behavior. Chamberlain et al. estimate that 
methane is now being added to the atmosphere at a rate between 0.5 and 
1.0 Gt per year, primarily from anaerobic fermentation of organic 
material in rice paddies, swamps, and tundras, plus enteric fermentation 
in the digestive tracts of ruminant animals. Anaerobic fermentation in 
the guts of termites, which contain cellulose-digesting symbiotic bac- 
teria, is probably also a significant source. Some methane is being 
added to the ocean-atmosphere system from vents in the rift zones of 
the ocean floor (Welhan and Craig, 1979) and perhaps of East Africa 
(Deuser et al., 1973). As we shall see, ocean sediments on the con- 
tinental slopes may be a relatively small source now but an important 
source in the future (MacDonald, 1982a) . 

Methane is removed from the lower atmosphere by a reaction with 
hydroxyl (HO) and is eventually oxidized to C0 2 . With the above 
estimate of the rate of input of 0.5 to 1.0 Gt per year, the residence 
time in the air should be between 5 and 10 years. Measurements indicate 
that the methane content of the air is increasing perhaps by 0.07 Gt 
per year, or about 1.4% per year, doubling in 70 years (Rassmussen and 
Khalil, 1981; Craig and Chou, 1982) . Part of this increase may be the 
direct result of such human actions as expansion of the area of rice 
paddies to meet the food needs of growing populations. A small part 
may represent release of methane from methane hydrates in continental 
slope sediments as the ocean responds to atmospheric warming. 

MacDonald (1982a) defines methane hydrate as a "type of clathrate in 
which methane and smaller amounts of ethane and other higher hydro- 
carbons are trapped within a cage of water molecules in the form of 
ice." Though it is not a .stoichiometric compound, about 6 mol of water 
are required for 1 mol of methane in the clathrate. Methane hydrates 
are stable at low temperatures and relatively high pressures (Claypool 
and Kaplan, 1974? Miller, 1974). They are found at depths between 200 
and 1000 m below the ground surface in permafrost (Kvenvolden and 



253 

McMenamin, 1980; Chersky and Makogon, 1970) and should be present near 
the surface of marine bottom sediments below water depths of 290 to 
more than 800 m f depending on bottom-water temperature. Within the 
sediments the thickness of the clathrate zone will be limited by the 
geothermal gradient of about 30C km"^ f which reflects heat conduc- 
tion from the interior of the Earth. 

As Bell (1982) has demonstrated, even with a CO 2 - induced rise in 
surface air temperatures of around 10 C, virtually none of the clathrate 
in permafrost would become unstable during the next several hundred 
years because the surface heating of the "frozen ground" would first 
have to penetrate and melt the permafrost in a 200-m-thick clathrate- 
free zone. The enthalpy (latent heat of fusion) of the ice in perma- 
frost would greatly slow the downward penetration of the heat wave. 

But with a rise in ocean-bottom temperatures, the uppermost layers 
of sediments would also become warmer and methane hydrates would become 
unstable in the upper limit of their depth range/ that is, about 300 m 
in the Arctic and about 600 m at low latitudes. 



3.5.2 Formation of Methane Clathrate in Continental Slope Sediments 

The quantity of clathrates that will be released from sediments under 
the seafloor as a result of ocean warming depends on the distribution 
of clathrates with depth and on their total abundance in the sediments. 
Estimates of total abundance by different authors differ by a factor of 
500 , from 10 3 to 5 x 10 5 Gt (MacDonald f 1982b) . If the methane 
locked up in clathrates were produced by anaerobic fermentation of 
organic matter in the sediments , one would expect that most oceanic 
clathrates would be found in deep semienclosed basins and on continental 
slopes, particularly on passive continental margins such as those on 
both sides of the Atlantic. 

The rate of sedimentation on continental slopes is relatively high 
(of the order of 10 to 20 cm/1000 years) , and samples taken from near 
the surface of the deposits are high in organic matter on the average 
about 2% of the dry weight (Trask, 1932) . This organic matter is some- 
times called "marine humus." It consists mainly of the partially 
decomposed tissues of marine plankton and nekton and to a lesser extent 
of the remains of terrestrial plants. On average, according to Trask, 
the carbon content of the organic matter is 56%; hence, organic carbon 
averages 1.12% of the dry weight of the uppermost layers of sediments.* 
The average density of the dry material is 2.6 g cnT^. 



*J. G. Erdman and colleagues of the Phillips Petroleum Company 
measured the organic carbon content of many samples collected by the 
Deep Sea Drilling Project from outer continental margins. The results 
were published in the Initial Reports of the Deep Sea Drilling Project 
(Volumes XXIV, XXXI, XXXVIII, XL, XLI, XLII, XLIII, XLIV, XLVII, 
XLVIII, L, and LXVI, published between 1975 and 1981, inclusive). 
Seventy-three samples of Quaternary to late Pliocene age from the 

(continued overleaf) 



254 

These deposits are very porous; cores of freshly collected mud 
usually contain two thirds water by volume. Thus, an average liter of 
mud from near the surface of the deposits will contain 650 g of water 
and 870 g of solids/ including silicate mineral grains, fragments of 
calcareous and siliceous skeletons, and shells, and about 17 g of 
organic matter containing close to 10 g of carbon. 

Because of the relatively high rates of deposition and the abundance 
of decomposable organic material, free oxygen in the interstitial water 
of these sediments is rapidly depleted as they are buried, and "reduc- 
ing" conditions prevail a short distance below the seafloor. The 
principal living organisms under these conditions are anaerobic bac- 
teria, which are able to carry out their metabolic activities in the 
absence of free oxygen. Dissolved sulfate in the interstitial water 
will first be reduced to sulfide, and a small fraction (less than 0.5 g 
per liter) of organic carbon will be oxidized to C0 2 . All the 
sulfate is usually depleted in the top meter of the sediments. Beneath 
this top layer, methane is produced (Claypool and Kaplan, 1974). Below 
water depths of 300 to 600 m, depending on bottom-water temperature, 
methane in excess of the quantity that can be dissolved in the 
interstitial water will be converted to methane hydrate as soon as it 
is formed. 

There are no measurements of the actual methane concentration in 
deep-sea muds in situ. In several cores from areas of rapid deposition 
collected by the Deep Sea Drilling Project (DSDP) , gas was observed 
escaping when the core liners were removed from the bore barrels on 
board Challenger. Some mud was ejected from the liners by the force of 
the escaping gas. Subsequent analysis showed this gas to be almost 
entirely methane (Mclver, 1974). Presumably, most of the methane 
escaped from the samples while they were being raised from the seafloor 
and handled on deck, but the remaining amounts were surprisingly 
high up to 15 mmol of methane per liter of interstitial water. 

The organic carbon content of DSDP sample with high remaining gas 
content after shipboard handling (presumably those in which the methane 
was originally present as a clathrate) ranged from 0.28 to 1.14% 
averaging 0.62% by dry weight (Mclver, 1974). Assuming that this 
organic carbon represents the residue of organic matter after methano- 
genesis has been completed, and that the proportion of residual carbon 
to carbon in methane produced is roughly the same (1:0.51) as in the 



(continued from overleaf) 

northern Indian Ocean, eastern Pacific, north and south Atlantic, and 
the Japan, Mediterranean, and Black Seas have an average organic carbon 
content of 1.4% of the dry weight of the sediments. The depths beneath 
the sediment surface ranged from less than 1 to 1000 m, with most of 
the samples being from 30 to 300 m below the top of the sediment; the 
average depth of the overlying water was around 2000 m. Presumably the 
measured organic carbon represents the residue after sulfate reduction 
and methanogenesis. Erdman believes that Trask's analysis of surface 
sediment gave low results because the samples were poorly preserved. 



255 

"biogas" digesters described by Makhijani and Poole (1975) , the cal- 
culated average methane content of these muds in situ is 0.42% by 
weight of dry sediment , or 3.6 g per liter of wet mud. The correspond- 
ing concentration of methane in the interstitial seawater would be 
about 330 mmol kg" 1 . 

The biochemical processes in the buried sediments are not well under- 
stood and may be quite different from those in methane-producing biogas 
digesters. J. G. Erdman (Bartlesville, Oklahoma, personal communica- 
tion) has pointed out that the microbial population in marine sediments 
drops rapidly with depth in the sediments and that the organic matter , 
unlike terrestrial biomass f is relatively lean in the hydrolyzable con- 
stituents of plant materials that ferment easily. Erdman is convinced 
that the mass of methane hydrate in marine sediments is larger than the 
amount calculated above. He believes that most of this methane was 
formed from sedimentary organic matter by thermolytic processes under 
heat and pressure at substantial depth in the sediments. The methane 
then migrated upward until it became trapped in the zone of methane 
hydrate stability in the upper sedimentary layers. This hypothesis has 
the significant advantage that it does not require the methane in the 
upper part of the hydrate zone to have formed since the last inter- 
glacial approximately 125,000 years ago, when subsurface ocean warming 
may have been as great as that expected with a doubling of atmospheric 
carbon dioxide. 

An estimate of the minimum concentration of methane can be made from 
the inferred existence of methane clathrate in muds from the Blake 
Plateau off the southeastern coast of the United States at a total 
depth (overlying water plus depth in the sediments) of about 4000 m 
(Stoll et al., 1971; Bryan, 1974). In order for a clathrate to form, 
the concentration of dissolved methane must have been close to 64-69 
mmol kg" 1 of interstitial water, which is the solubility of methane 
at a hydrostatic pressure of 400 atm (Claypool and Kaplan, 1974) . This 
concentration is about 20% of that calculated above by comparison with 
observed methane production in biogas digesters. 

For our present purposes, we may assume that the concentration of 
methane in continental slope muds is halfway between these two esti- 
mates, say 200 mmol kg" 1 of interstitial water, or 2.2 g of methane 
per liter of mud. If all of this methane is present as clathrates, 
about 1200 mmol kg" 1 of water will also be in the same state or 21.6 
g kg" 1 of interstitial water. This is 3.2% of the water in an 
average mud. 

Miller (1974) shows that in seawater, the minimum hydrostatic 
pressure (P) at which methane hydrate is stable between temperatures 
(T) of and 10 C is given by 

Iog 10 P (atmospheres) 1.4613 + 0.0416T + 2.93 x 10" 4 (T) 2 , 

where P (atmospheres) is the partial pressure of methane, which is 
equal to hydrostatic pressure when the water is saturated with methane. 
The water depths below which methane hydrate in the uppermost layers of 
marine bottom sediments will be stable at different temperatures can be 
calculated from this equation, with the simplifying assumption that 



256 

hydrostatic pressure in the ocean increases by approximately 1 atm for 
each 10 m of depth: 

Bottom-Water Minimum Depth of 

Temperature (C) Clathrate Stability (m) 

289 

1 319 

2 351 

3 388 

4 429 

5 475 

6 528 

7 588 

8 650 

9 724 
10 807 



3.5.3 Effect of Carbon Dioxide- Induced Warming on Continental Slope 
Clathrates 

With carbon dioxide- induced warming of the atmosphere, ocean surface 
temperatures will rise by a nearly equal amount/ and heat will be 
carried downward by advection and eddy diffusion into the subsurface 
water layers. For a doubling of atmospheric CC>2 and the expected 
increase in other "greenhouse gases," with an assumed sensitivity of 
global average temperature of 3C for a CC>2 doubling, the temperature 
increase in different latitudes at the water depths below which methane 
hydrate is stable at present can be estimated (see this volume, Chapter 
8, Section 8.3). The corresponding increases in Clathrate stability 
depths are shown in Table 3.6. 

The depth of melting of the Clathrate below the sediment-water 
interface at any time after the bottom-water temperature is raised will 
be much smaller than the depth at which warming will occur in the 
absence of Clathrate. When bubbles of methane are formed, the latent 
heat (enthalpy) of vaporization of the methane must be added to that 
for melting of the water-ice in the Clathrate. Miller (1974) has 
calculated that with 6 mol of water per mol of methane, the combined 
enthalpies are 120 cal g" 1 of H 2 in the clathrate. Because most 
of the water in the sediment remains liquid even after clathrate has 
formed, we can compute a "virtual" latent heat, L, required for the 
depth of wetting to advance downward by 1 cm. 

L = 0.032 x 120 + 0.968 x 1 * 4.8 cal g" 1 of H 2 0. 

The problem of the rate of downward advance of the depth of melting 
has been solved by W. H. Munk (La Jolla, California, personal 
communication) . He shows that the depth h (cm) of the melted zone at 
time t (sec) after an instantaneous rise in water temperature at the 
sediment-water interface is given approximately by, 



257 
h = a (Kt) 1 / 2 , 



where ic(cm 2 sec" 1 ) - K/p C; K is thermal conductivity of wet 
sediment, in cal sec" 1 cm"" 1 C~ 1 ? p is density of wet 
sediments f g cm" 3 ? C is specific heat, cal g" 1 C~ 1 , 



and 



a(dimensionless) = -^ (T - T ) I 1 / 2 

L L 1 m J ; 



L is "virtual" enthalpy of melting for clathrate in sediments, cal 
g" 1 of H 2 in both clathrate and liquid fractions of interstitial 
water; T - T m is temperature at the sediment-water interface minus 
temperature of melting of clathrate. 

With the following approximate numerical values: K = 4 x 10" 3 
(calculated from average geothermal heat flow through the seafloor and 
estimated temperature gradient in sediment: K 1.3 x 10~ 6 -r 3 x 
10" 4 ),p = 1.54, T! - T m 1, C 1, L 4.8,K = 2.6 x 
10" 3 , anda 0.65, then h = O.OSSKt) 1 / 2 cm. 

After an instantaneous rise of bottom-water temperature of 1C, the 
depth of melting would advance downward 18.6 m beneath the sediment- 
water interface during 100 years, or an average of 18.6 cm yr" 1 . An 
average of 0.041 g of methane per cm 2 of seafloor should be released 
each year in the area where clathrates have become unstable. 

The rate of exchange of dissolved materials between the sediments 
and the overlying water is very slow, and consequently methane will 
probably not diffuse significantly out of the sediments until its con- 
centration becomes high enough to form bubbles of the gas. According 
to Miller (1974) , bubbles of methane will form at a hydrostatic pres- 
sure of 400 atm when the concentration exceeds 350 mmol liter" 1 . 
Provided the relation between the concentration required for bubble 
formation and hydrostatic pressure is approximately linear, the required 
concentration at about 500 m will be 44 mmol. With our assumed concen- 
tration of 200 mmol kg" 1 of interstitial water, close to 80% of the 
methane released from clathrate should escape from the mud in bubbles 
and should rise rapidly to the sea surface before it can be oxidized in 
the water. 



3*5.4 Future Rate of Methane Release from Sedimentary Clathrates 

From Table 3.6 we see that the average depth interval on the continental 
slope over which the clathrates will become unstable with a CO 2 doub- 
ling is about 100 m. Using Kossina's 1921 estimate (see also Menard 
and Smith, 1966) that the area of seafloor between 200 and 1000 m is 
15.5 x 10 16 cm 2 , and assuming that the depths of continental slopes 
increase linearly in this depth range, the area in which clathrates 
will become unstable is 1.94 x 10 16 cm 2 , and the total quantity of 
methane released annually as bubbles in the bottom muds should be 0.8 x 
0.041 x 1.94 x 10 16 g, or, 0.64 Gt (10 9 metric tons). With an 
atmospheric residence time of 5 to 10 years, the quantity of methane in 



258 



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259 

the air could rise by 3.2 to 6.4 Gt f two thirds to four thirds of the 
present amount. This quantity would be in addition to most of the 
present rate of increase of about 7 Gt/century. The corresponding 
increase in global average surface temperature from the methane "green- 
house" effect toward the latter part of the twenty-first century couX 
be 0.65 to 0.8C if we accept the estimate of Lacis et al. (1981) for 
the equilibrium warming resulting from a doubling of methane, and 1.5 
to 1.8C, using the estimate of Chamberlain et al. (1982). As the CO 9 
produced by oxidation of the methane accumulates in the air, there will 
be a slight further rise in temperature, of the order of 0.1 to 2C 
in a hundred years. 

Bell (1982) assumed that a C0 2 -induced warming of the Arctic Ocean 
would release methane hydrates during 100 years from the top 40 m of 
the bottom sediments over half the area between water depths of 280 and 
370 m. The estimated bottom area between these depths in the Arctic 
Ocean and Arctic Mediterranean is 0.240 million knr or about 1.5% of 
the area between 200 and 1000 m in the world ocean (Menard and Smith, 
1966). Bell assumes that 10,000 Gt of C as methane hydrate are 
uniformly distributed in the top 250 m of the sediments over the world 
ocean area between 200 and 1000 m. The total release from the Arctic 
Ocean regions in 100 years would then be 12 Gt of C or 0.12 Gt/yr. 
(Bell's figure of 8 Gt of C/yr is an obvious computational error.) 

Obviously, the uncertainties in calculations of release of methane 
from continental slope sediments are so great that the results cannot 
be thought of as a projection for the future. But it is equally 
obvious that an extensive sediment sampling program on continental 




ONSHORE 
OFFSHORE 



FIGURE 3.19 Reported occurrences of natural gas hydrates. (Updated 
from Kvenvolden and McMenamin, 1980.) 



260 

slopes throughout the world should be undertaken to determine the 
depth, thickness, and distribution of methane hydrate clathrates, 
especially where oceanfloor temperatures and depths are such that 
methane release is likely from ocean warming during the next century. 
A small release of methane clathrates may already be taking place as a 
consequence of the estimated 0.5C increase of global air temperature 
(Hansen et al., 1982; Jones et al., 1982; Weller et al., this volume/ 
Chapter 5, Section 5.2) during the last century, and the probable 
increase of ocean-bottom temperatures by 0.1-0.2C from eddy diffusion 
and advection down to 500 m. 

Some indications that clathrates may be widespread and abundant in 
ocean sediments are given in Figure 3.19 (from MacDonald, 1982b) . This 
shows the locations in the continental slopes of the ocean floor and in 
the Black and Caspian Seas, in which the existence of methane 
clathrates has been inferred from high gas contents in cores of the 
Deep Sea Drilling Project or in which their presence is suspected from 
acoustic reflections from a layer below the sediment surface that 
parallels the ocean-bottom topography (Bryan, 1974; Stoll et al., 
1971) . It is believed that these are reflections from the bottom of 
the methane clathrate zone, although other explanations are possible. 
The likelihood of the widespread occurrence of clathrates in 
continental slope sediments gives force to our argument that a 
systematic survey should be made in an attempt to determine their 
abundance and distribution. 



References 

Bell, P. R. (1982) . Methane hydrate and the carbon dioxide question. 

In Carbon Dioxide Review; 1982, W. C. Clark, ed. Oxford U. Press, 

New York, pp. 401-406. 
Bryan, G. M. (1974). In situ indications of gas hydrate. In Natural 

Gases in Marine Sediments , I. R. Kaplan, ed. Plenum, New York, pp. 

151-170. 
Chamberlain, J. W., H. M. Foley, G. J. MacDonald, and M. A. Ruderman 

(1982) . Climate effects of minor atmospheric constituents. In 

Carbon Dioxide Review: 1982 , W. C. Clark, ed. Oxford U. Press, New 

York, pp. 255-277. 
Chersky, N., and Y. Makogon (1970). Solid gas world reserves are 

enormous. Oil and Gas Inter nat. 10 ;8. 
Claypool, G. E., and I. R. Kaplan (1974). The origin and distribution 

of methane in marine sediments. In Natural Gases in Marine 

Sediments, I. R. Kaplan, ed. Plenum, New York, pp. 99-140. 
Craig, H., and C; C. Chou (1982). Methane, the record in polar ice 

cores . Geophys. Res. Lett. 9; 1221-1224 . 
Deuser, W. , E. Degens, G. Harvey, and M. Rubin (1973). Methane in Lake 

Kiva: new data bearing on its origin. Science 181; 51-53. 
Hansen, J. , D. Johnson, A. Lac is, S. Lebedeff, P. Lee, D. Rind, and G. 

Russell (1981) . Climatic impact of increasing atmospheric carbon 

dioxide. Science 213;957-966. 



261 

Jones, P. D., R. M. L. Wigley, and P. M. Kelly (1982). Mon. Weather 

Rev, 110; 59-70, 
Kossina, E. (1921). Die Tiefen des Weltmeeres. Berlin U. Inst. fur 

Meereskunde. Veroff. N.F., A Geogr. Naturwiss. Heft 9, 70 pp. 
Kvenvolden, K. A., and M. A. McMenamin (1980). Hydrates of natural 

gas: a review of their geologic occurrence. U.S. Geological Survey 

Circ. 825. U.S. Dept. of the Interior, Washington, D.C. 
Lacis, A., J. Hansen, P. Lee, T. Mitchell, and S. Lebedeff (1981). 

Greenhouse effect of trace gases, 1970-1980. Geophys. Res. Lett. 

8.:1035-1038. 
MacDonald, G. J., ed. (1982a) . The Long-Term Impacts of Increasing 

Atmospheric Carbon Dioxide Levels. Ballinger, Cambridge, Mass., 273 

pp. 
MacDonald, G. J. (1982b) . The many origins of natural gas. Paper 

presented May 1982 at Deep Source Gas Workshop sponsored by 

Morgantown Energy Technology Center. U.S. Dept. of Energy. 
Mclver, R. D. (1974). Hydrocarbon gas (methane) in canned Deep Sea 

Drilling Project core samples. In Natural Gases in Marine 

Sediments, I. R. Kaplan, ed. Plenum, New York, pp. 63-70. 
Makhijani, A., and A. Poole (1975). Energy and Agriculture in the 

Third World, Appendix B, Biogasif ication. Ballinger, Cambridge, 

Mass., pp. 143-160. 
Menard, H. W., and S. M. Smith (1966). Hypsometry of ocean basin 

provinces . J. Geophys. Res. 71; 4305-4321 . 
Miller, S. R. (1974). The nature and occurrence of clathrate 

hydrates. In Natural Gases in Marine Sediments, I. R. Kaplan, ed. 

Plenum, New York, pp. 151-170. 
Rasmussen, R. A., and M. A. K. Khalil (1981). Atmospheric methane 

(CH 4 ) : trends and seasonal cycles. J. Geophys. Res. 86:9826-9832. 
Stoll, R. D., J. Ewing, and G. Bryan (1971). Anomalous wave velocities 

in sediments containing gas hydrates. J. Geophys* Res. 76(8) :2090. 
Trask, P. D. (1932). Origin and environment of source sediments of 

petroleum. American Petroleum Institute, Gulf Publ. Co., Houston, 

Tex., reprinted 1982, 332 pp. Quoted in H. Sverdrup et al. (1942), 

The Oceans. Prentice-Hall, New York, pp. 1009-1015. 
Welhan, J., and H. Craig (1979). Methane and hydrogen in East Pacific 

Rise hydrothermal fluids. Geophys. Res. Lett. 6:829-831. 



262 

3.6 SENSITIVITY STUDIES USING CARBON CYCLE MODELS 
Lester Machta 

Sensitivity studies using carbon cycle models provide a way of 
estimating uncertainties in model predictions and aid in distinguishing 
between those factors in the model that require improvement and those 
whose uncertainty makes little difference in a final answer. On the 
other hand f sensitivity studies cannot identify defects that are incor- 
porated in the model and for which no sensitivity analysis is possible. 
Furthermore, a model based on the present physical and biological world 
usually assumes that future behavior of that world can be derived from 
its past and current status. One should remember that all models of 
the real world limit their treatment to one or at most a very few 
forcing functions. In reality many other factors may also produce 
relevant changes. 

There are a wide variety of carbon cycle models currently available 
into which one may enter values for fossil fuel or other C02 sources. 
These models have in common three reservoirs: the atmosphere/ the 
oceans, and the biosphere. They also limit themselves to exchange 
processes that act on a time scale less than several thousand years; 
that is, many geologic processes are omitted. Each model subdivides 
the reservoirs and transports carbon and its isotopes in different 
ways. All models inject the past releases of fossil fuel C02 and, in 
a few cases other sources of C0 2 , into the atmospheric reservoir and 
try to reconstruct the growth of C0 2 concentration in air as found 
at, say, Mauna Loa Observatory after 1958. In some instances, other 
observations, not involved in model development, may also be used to 
validate the model* 

Few, if any, models of the carbon cycle would likely be published 
unless there were some agreement with the Mauna Loa C0 2 record. 
Thus, using predictions from the preindustrialized period to the 
1958-1982 interval may not be useful for sensitivity studies. Rather, 
predictions into the future, as long as they use the same projected 
C0 2 releases, are a better approach to sensitivity. Note that this 
in no way implies that the future emissions are known; merely that they 
represent convenient and realistic numbers to use for sensitivity studies. 



3.6.1 Comparison among Different Models 

Killough and Emanuel (1981) have compared the projections made from two 
simple ocean box models and three other more complex ocean layer models. 
In each case, plausible parameters for the reservoirs, which include an 
atmosphere and biosphere as well as the oceans, are used to determine 
the size and exchange between reservoirs. Figure 3.20 illustrates the 
projection to year 2275. The topmost curve labeled "cumulative release 
+290 ppmv" represents the assumed curve of the input of C0 2 into the 
atmosphere expressed in units of the atmospheric concentration as 
though all the CO 2 remained airborne and were uniformly mixed in the 
air. The input scenario and the models suggest a peak concentration in 
the air about six times the preindustrial concentration of 290 ppmv. 



FIGURE 3.20 Atmospheric 
response levels of C02 
based on cumulative 
release as shown. Net 
exchange with terrestrial 
biota was assumed to be 
zero after an asymptotic 
transition period within 
the decade after 1975. 
(From Ki Hough and 
Emmanuel, 1981.) 



i 

8" 

o 

DC 
LJJ 

1C 



i 



263 
<x10 2 ) 
26 
22 
18 
14 
10 
6 



I I I I I I I I I I 
Cumulative release + 290 ppmv 



Atmospheric response curves 
for the five models 




I I I 



I I I 



1975 



2050 



2125 2200 
YEAR 



2275 



At this peak/ the range between the minimum and maximum concentrations 
from the five models was 350 ppmv or about 18% of the peak concentra- 
tion. Were one to select the average among five models, the maximum 
deviation of the lowest and highest concentration from the five models 
would differ from the mean by less than 10%. 

Laurmann and Spreiter (1983) have compared three simple box-type 
models (2, 3, and 4 boxes or carbon reservoirs). They conclude that 
not only are predictions of future concentrations from these three 
models sufficiently similar/ but the use of a single airborne fraction 
(they use the term "retention fraction") yields similar results as the 
box models. Two exceptions are noted. First/ the models do diverge if 
the growth rate of C0 2 emissions is small/ an exponential growth rate 
of less than 1.5% per year. Second/ if the transfer to the deep oceans 
is much faster than used in the models then all bets are off (i.e./ the 
models do not represent nature) . It is noted that problems will arise 
if there has been significant deforestation C0 2 during the past 
several decades; the three models assume none. 



3.6.2 Comparison of Parameters within a Single Model 

As part of the analysis of their carbon cycle model/ Enting and Pearman 
(1982) have undertaken a sensitivity study. Although the model does 
not allow for geographical variation in any of the three major reser- 
voirs (atmosphere/ oceans/ and biosphere)/ it contains many features 
beyond those of the other models described in this review of sensitiv- 
ity studies. Virtually all of the parameters (or observations leading 
to the choice of a value for a parameter) were studied. The conclu- 
sions drawn by Enting and Pearman are: "The sensitivity analysis . . . 



265 

half of about 600 x lO^- 5 g of C) would result in an increase of 
perhaps 75 ppmv in a predicted value of about 1,000 ppmv in the year 
2100. 

On the other hand, if significant deforestation is currently in 
progress, and has been for the past several decades, irrespective of 
future deforestation, the issue is different from that described in the 
above paragraph. If, for example, the deforestation CO2 were about 
as large as that from fossil fuel sources, the current models would 
fail to reproduce the observed atmospheric C02 growth after 1958. 
The models would likely have to be modified since no reasonable adjust- 
ment of the parameters will allow a good fit of predictions to observa- 
tions after 1958. The airborne fraction, the ratio of atmospheric 
increase in a year to the net amount added to the atmosphere, would be 
calculated to drop to about 0.3 from a value of almost 0.6. Instead of 
an increase of predicted concentration from 340 to 1000 ppmv, the 
increase might be only from 340 to 670 ppmv to the year 2100. 



3.6.4 Conclusion 

It may be worth noting what the limited survey of sensitivity studies 
does and does not reveal. The studies do suggest that if the carbon 
cycle models are accepted as valid representations of reality, reason- 
able variations in the numerical values of the parameters do not appear 
to affect significantly the predictions of future concentrations of 
atmospheric C0 2 . Research that accepts the physics, chemistry, and 
biology of the existing models but tries simply to refine the parameters 
may not be so effective as research in other aspects of the carbon 
dioxide issue. On the other hand, the sensitivity studies reveal 
nothing about the ability of the models to represent nature either 
today or in the future except possibly indirectly. 

The guidance provided by sensitivity studies suggests the need for 
research in those aspects of the carbon cycle likely to make a 
difference for predicting future atmospheric C0 2 concentrations. 



References 

Enting, I. G., and G. I. Pearman (1982). Description of a 

one-dimensional global carbon cycle model. Paper No. 42. Div. of 
Atmos. Phys., CSIRO, Australia. 

Keeling, C. D., and R. B. Bacastow (1977). Impact of industrial gases 
on climate. Energy and Climate. Geophysics Research Board, 
National Research Council, National Academy Press, Washington, D.C. 

Killough, G. G. , and W. R. Emanuel (1981). A comparison of several 
models of carbon turnover in the ocean with respect to their 
distributions of transit time and age, and responses to atmospheric 
C0 2 and 14 C. Tellus 33; 274-290. 

Laurmann, J. A., and J. R. Spreiter (1983). The effects of carbon 
cycle model error in calculating future atmospheric carbon dioxide 
levels. Climatic Change 5; 145-175. 



4 Effects on Climate 



4.1 EFFECTS OF CARBON DIOXIDE 
Joseph Smagorinsky 

4-1*1 Excerpts from "Charney" and "Smagorinsky" Reports 

From the beginning of our Committee's work it was clear that at least 
one aspect of the C0 2 issue would require continuing attention: the 
linkage between increases in atmospheric C0 2 and changes in climate. 
This question had been addressed in 1979 by a panel chaired by the late 
Jule Charney (Climate Research Board, 1979) , and I was asked to lead a 
similar group to take a second look in the light of subsequent research 
results. Our report, Carbon Dioxide and Climate; A Second Assessment 
(C0 2 /Climate Review Panel, 1982) , was to its authors both reassuring 
and disappointing. On one hand, we found no reasons to dispute the 
judgments of the Charney group: increased carbon dioxide can poten- 
tially produce climate changes sufficiently significant to merit con- 
cern. On the other, we continued to find large uncertainties in the 
timing, magnitude, character, and spatial distribution of these changes. 
At this writing, the results of our study and its predecessor still 
represent in my view a sober, balanced, and responsible consensus on 
the climatic implications of increased CO 2 . Thus, the summarized 
conclusions of our study (and the earlier Charney report) are repro- 
duced below. However, in the year that has elapsed since their 
formulation, the scientific issues have been further illuminated by 
research, and I will append in Section 4.1.2 some comments as an 
epilogue to the CO ^Climate Review Panel report. 



266 



267 



1 

Summary and Conclusions 



We have examined the principal attempts to simulate the effects of increased 
atmospheric CO 2 on climate. In doing so, we have limited our considerations 
to the direct climatic effects of steadily rising atmospheric concentrations of 
C0 2 and have assumed a rate of CO 2 increase that would lead to a doubling 
of airborne concentrations by some time in the first half of the twenty-first 
century. As indicated in Chapter 2 of this report, such a rate is consistent 
with observations of C0 2 increases in the recent past and with projections 
of its future sources and sinks. However, we have not examined anew the 
many uncertainties in these projections, such as their implicit assumptions 
with regard to the workings of the world economy and the role of the 
biosphere in the carbon cycle. These impose an uncertainty beyond that 
arising from our necessarily imperfect knowledge of the manifold and 
complex climatic system of the earth. 

When it is assumed that the C0 2 content of the atmosphere is doubled and 
statistical thermal equilibrium is achieved, the more realistic of the modeling 
efforts predict a global surface warming of between 2C and 3.5C, with 
greater increases at high latitudes. This range reflects both uncertainties in 
physical understanding and inaccuracies arising from the need to reduce the 
mathematical problem to one that can be handled by even the fastest avail- 
able electronic computers. It is significant, however, that none of the model 
calculations predicts negligible warming. 

The primary effect of an increase of C0 2 is to cause more absorption of 
thermal radiation from the earth's surface and thus to increase the air tem- 
perature in the troposphere. A strong positive feedback mechanism is the 
accompanying increase of moisture, which is an even more powerful absorber 

1 

From Carbon Dioxide and Climate; A Scientific 
Assessment , Report of an Ad Hoc Study Group on 
Carbon Dioxide and Climate, National Research 
Council, 1979. 



268 



CARBON DIOXIDE AND CLIMATE: A SCIENTIFIC ASSESSMENT 



of terrestrial radiation. We have examined with care all known negative feed- 
back mechanisms, such as increase in low or middle cloud amount, and have 
concluded that the oversimplifications and inaccuracies in the models are not 
likely to have vitiated the principal conclusion that there will be appreciable 
warming. The known negative feedback mechanisms can reduce the warming, 
but they do not appear to be so strong as the positive moisture feedback. We 
estimate the most probable global warming for a doubling of C0 2 to be near 
3C with a probable error of 1.5C. Our estimate is based primarily on our 
review of a series of calculations with three-dimensional models of the global 
atmospheric circulation, which is summarized in Chapter 4. We have also re- 
viewed simpler models that appear* to contain the main physical factors. 
These give qualitatively similar results. 

One of the major uncertainties has to do with the transfer of the increased 
heat into the oceans. It is well known that the oceans are a thermal regulator, 
warming the air in winter and cooling it in summer. The standard assumption 
has been that, while heat is transferred rapidly into a relatively thin, well- 
mixed surface layer of the ocean (averaging about 70 m in depth), the trans- 
fer into the deeper waters is so slow that the atmospheric temperature reaches 
effective equilibrium with the mixed layer in a decade or so. It seems to us 
quite possible that the capacity of the deeper oceans to absorb heat has been 
seriously underestimated, especially that of the intermediate waters of the 
subtropical gyres lying below the mixed layer and above the main thermo- 
cline. If this is so, warming will proceed at a slower rate until these inter- 
mediate waters are brought to a temperature at which they can no longer 
absorb heat. 

Our estimates of the rates of vertical exchange of mass between the mixed 
and intermediate layers and the volumes of water involved give a delay of the 
order of decades in the time at which thermal equilibrium will be reached. 
This delay implies that the actual warming at any given time will be appre- 
ciably less than that calculated on the assumption that thermal equilibrium is 
reached quickly. One consequence may be that perceptible temperature 
changes may not become apparent nearly so soon as has been anticipated. We 
may not be given a warning until the C0 2 loading is such that an appreciable 
climate change is inevitable. The equilibrium warming will eventually occur; it 
will merely have been postponed. 

The warming will be accompanied by shifts in the geographical distribu- 
tions of the various climatic elements such as temperature, rainfall, evapora- 
tion, and soil moisture. The evidence is that the variations in these anomalies 
with latitude, longitude, and season will be at least as great as the globally 
averaged changes themselves, and it would be misleading to predict regional 
climatic changes on the basis of global or zonal averages alone. Unfortunately, 
only gross globally and zonally averaged features of the present climate can 



269 



Summary and Conclusions 



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now be reasonably well simulated. At present, we cannot simulate accurately 
the details of regional climate and thus cannot predict the locations and 
intensities of regional climate changes with confidence. This situation may be 
expected to improve gradually as greater scientific understanding is acquired 
and faster computers are built. 

To summarize, we have tried but have been unable to find any overlooked 
or underestimated physical effects that could reduce the currently estimated 
global warmings due to a doubling of atmospheric CO 2 to negligible propor- 
tions or reverse them altogether. However, we believe it quite possible that 
the capacity of the intermediate waters of the oceans to absorb heat could 
delay the estimated warming by several decades. It appears that the warming 
will eventually occur, and the associated regional climatic changes so impor- 
tant to the assessment of socioeconomic consequences may well be signifi- 
cant, but unfortunately the latter cannot yet be adequately projected. 




270 



Summary of Conclusions and 
Recommendations 



For over a century, concern has been expressed that increases in atmospheric 
carbon dioxide (CO 2 ) concentration could affect global climate by changing 
the heat balance of the atmosphere and Earth. Observations reveal steadily 
increasing concentrations of CO 2 , and experiments with numerical climate 
models indicate that continued increase would eventually produce significant 
climatic change. Comprehensive assessment of the issue will require projection 
of future CO 2 emissions and study of the disposition of this excess carbon 
in the atmosphere, ocean, and biota; the effect on climate; and the implications 
for human welfare. This study focuses on one aspect, estimation of the effect 
on climate of assumed future increases in atmospheric CO 2 . Conclusions are 
drawn principally from present-day numerical models of the climate system. 
To address the significant role of the oceans, the study also makes use of 
observations of the distributions of anthropogenic tracers other than CO 2 . 
The rapid scientific developments in these areas suggest that periodic 
reassessments will be warranted. 

The starting point for the study was a similar 1979 review by a Climate 
Research Board panel chaired by the late Jule G. Charney. The present study 
has not found any new results that necessitate substantial revision of the 
conclusions of the Charney report. 



SIMPLIFIED CLIMATE MODELS AND EMPIRICAL STUDIES 

Numerical models of the climate system are the primary tools for investigating 
human impact on climate. Simplified models permit economically feasible 



From Carbon Dioxide and Climate; A Second 
Assessment, Report of the COo/Climate Revipw 
Panel f National Research Council, National Academy 
Press, 1982. 



271 



2 CARBON DIOXIDE AND CLIMATE: A SECOND ASSESSMENT 

analyses over a wide range of conditions. Although they can provide only 
limited information on local or regional effects, simplified models are valuable 
for focusing and interpreting studies performed with more complete and 
realistic models. The sensitivity of global-mean temperature to increased 
atmospheric CO 2 estimated from simplified models is generally consistent 
with that estimated from more complete models. 

The effects of increased CO 2 are usually stated in terms of surface 
temperature, and models of the energy balance at the surface are often 
employed for their estimation. However, changes in atmospheric CO 2 actually 
affect the energy balance of the entire climate system. Because of the strong 
coupling between the surface and the atmosphere, global-mean surface 
warming is driven by radiative heating of the entire surface-atmosphere 
system, not only by the direct radiative heating at the surface. 

Theoretical and empirical studies of the climatic effects of increased CO 2 
must properly account for all significant processes involved, notably changes 
in the tropospheric energy budget and the effects of ocean storage and 
atmospheric and oceanic transport of heat. For example, studies of the 
isolated surface energy balance or local observational studies of the transient 
response to short-term radiative changes can result in misleading conclusions. 
Otherwise, such studies can grossly underestimate or, in some instances, 
overestimate the long-term equilibrium warming to be expected from increased 
CO 2 . Surface energy balance approaches and empirical studies are fully 
consistent with comprehensive climate models employed for CO 2 sensitivity 
studies, provided that the globally connected energy storage and transport 
processes in the entire climate system are fully accounted for on the appropriate 
time scales. Indeed, empirical approaches to estimating climatic sensitivity 
particularly those employing satellite radiation budget measurements should 
be encouraged. 



ROLE OF THE OCEANS 

The heat capacity of the upper ocean is potentially great enough to slow 
down substantially the response of climate to increasing atmospheric CO 2 - 
The upper ocean will affect both the detection of CO 2 -induced climatic 
changes and the assessment of their likely social implications. The thermal 
time constant of the atmosphere coupled to the wind-mixed layer of the ocean 
is only 2-3 years. The thermal time constant of the atmosphere coupled to 
the upper 500 m of the ocean is roughly 10 times greater, or 20-30 years. 
On a time scale of a few decades, the deep water below 500 m can act as a 
sink of heat, slowing the rise of surface temperature. However, tracer data 
indicate that the globally averaged mixing rate into the deep ocean appears 



272 



Summary of Conclusions and Recommendations 3 

to be too slow for it to be of dominant importance on a global scale for time 
scales less than 100 years. 

The lagging ocean thermal response may cause important regional 
differences in climatic response to increasing CO 2 - The response in areas 
downwind from major oceans will certainly be different from that in the 
interior of major continents, and a significantly slower response to increasing 
CO 2 might be expected in the southern hemisphere. The role of the ocean in 
time-dependent climatic response deserves special attention in future modeling 
studies, stressing the regional nature of oceanic thermal inertia and atmo- 
spheric energy-transfer mechanisms. 

Progress in understanding the ocean' s role must be based on a broad 
program of research: continued observations of density distributions, tracers, 
heat fluxes, and ocean currents; quantitative elucidation of the mixing 
processes potentially involved; substantial theoretical effort; and development 
of models adequate to reproduce the relative magnitudes of a variety of 
competing effects. The problems are difficult, and complete success is 
unlikely to come quickly. Meanwhile, partially substantiated assumptions 
like those asserted here are likely to remain an integral part of any assessment. 
In planning the oceanographic field experiments in connection with the World 
Climate Research Program, particular attention should be paid to improving 
estimates of mixing time scales in the main thermocline. 

Present knowledge of the interaction of sea-ice formation and deep-water 
formation is still rudimentary, and it will be difficult to say even qualitatively 
what role sea ice will play in high-latitude response and deep-water formation 
until the climatic factors that control the areal extent of polar pack ice in the 
northern and southern hemispheres are known. Field experiments are required 
to gain fundamental observational data concerning these processes. 



CLOUD EFFECTS 

Cloud amounts, heights, optical properties, and structure may be influenced 
by CO 2 -induced climatic changes. In view of the uncertainties in our 
knowledge of cloud parameters and the crudeness of cloud-prediction schemes 
in existing climate models, it is premature to draw conclusions regarding 
the influence of clouds on climate sensitivity to increased CO 2 . Empirical 
approaches, including satellite-observed radiation budget data, are an impor- 
tant means of studying the cloudiness-radiation problem, and they should be 
pursued. 

Simplified climate models indicate that lowering of albedo owing to 
decreased areal extent of snow and ice contributes substantially to C0 2 
warming at high latitudes. However, more complex models suggest that 



273 



4 CARBON DIOXIDE AND CLIMATE: A SECOND ASSESSMENT 

increases in low-level stratus cloud cover may at least partially offset this 
decrease in albedo. In view of the great oversimplification in the calculation 
of clouds in climate models, these inferences must be considered tentative. 



OTHER TRACE GASES 

Although the radiative effects of trace gases (nitrous oxide, methane, ozone, 
and chlorofluoromethanes) are in most instances additive, their concentrations 
can be chemically coupled. The climatic effects of alterations in the con- 
centrations of trace gases can be substantial. 

Since trace-gas abundances might change significantly in the future because 
of anthropogenic emissions or as a consequence of C0 2 -initiated climatic 
changes, it is important to monitor the most radiatively significant trace 
gases. 



ATMOSPHERIC AEROSOLS 

Atmospheric aerosols are a potentially significant source of climate varia- 
bility, but their effects depend on their composition, size, and vertical-global 
distributions. Stratospheric aerosols consisting mainly of aqueous sulfuric 
acid droplets, which persist for a few years following major volcanic eruptions, 
can produce a substantial, but temporary, reduction in global surface 
temperature and can explain much of the observed natural climatic variability. 
While stratospheric aerosols may contribute to the infrared greenhouse effect, 
their net influence appears to be surface cooling. 

The climatic effect of tropospheric aerosols sulfates, marine salts, and 
wind-blown dust is much less certain, in part because of inadequate 
observations and understanding of the optical properties. Although anthro- 
pogenic aerosols are particularly noticeable in regions near and ddwnwind 
of their sources, there does not appear to have been a significant long-term 
increase in the aerosol level in remote regions of the globe other than possibly 
the Arctic. The climatic impact of changes in anthropogenic aerosols, if they 
occur, cannot currently be determined. One cannot even conclude that 
possible future anthropogenic changes in aerosol loading would produce 
worldwide heating or cooling, although carbon-containing Arctic aerosol 
definitely causes local atmospheric heating. Increased tropospheric aerosols 
could also influence cloud optical properties and thus modify cloudiness- 
radiation feedback. This possibility requires further study. 



274 



Summary of Conclusions and Recommendations 



THE LAND SURFACE 

Land-surface processes also influence climate, and the treatment of surface 
albedo and evapotranspiration in climate models influences the behavior of 
climate models. Land-surface processes largely depend on vegetatioh coverage 
and may interact with climatic changes in ways that are as yet poorly 
understood. 



VALIDATION OF CLIMATE MODELS 

Mathematical-physical models, whether in a highly simplified form or as 
elaborate formulations of the behavior and interactions of the global atmo- 
sphere, ocean, cryosphere, and biomass, are generally considered to be the 
most powerful tools yet devised for the study of climate. Our confidence in 
them comes from tests of the correctness of the models' representation of 
the physical processes and from comparisons of the models' responses to 
known seasonal variations. Because decisions of immense social and economic 
importance may be made on the basis of model experiments, it is important 
that a comprehensive climate-model validation effort be pursued, including 
the assembly of a wide variety 'of observational data specifically for model 
validation and the development of a validation methodology. 

Validation of climate models involves a hierarchy of tests, including checks 
on the internal behavior of subsystems of the model. The parameters used 
in comprehensive climate models are explicitly derived, as much as possible, 
from comparisons with observations and/or are derived from known physical 
principles. Arbitrary adjustment or tuning of climate models is therefore 
greatly limited. 

The primary method for validating a climate model is to determine how 
well the model-simulated climate compares with observations. Comparisons 
of simulated time means of a number of climatic variables with observations 
show that modern climate models provide a reasonably satisfactory simulation 
of the present large-scale global climate and its average seasonal changes. 

More complete validation of models depends on assembly of suitable data, 
comparison of higher-order statistics, confirmation of the models' represen- 
tation of physical processes, and verification of ice models. 

One test of climate theory can be obtained from empirical examination of 
other planets that in effect provide an ensemble of experiments over a variety 
of conditions. Observed surface temperatures of Mars, Earth, and Venus 
confirm the existence, nature, and magnitude of the greenhouse effect. 

Laboratory experiments on the behavior of differentially heated rotating 
fluids have provided insights into the hydrodynamics of the atmosphere and 
ocean circulations and can contribute to our understanding of processes such 



275 



6 CARBON DIOXIDE AND CLIMATE: A SECOND ASSESSMENT 

as small-scale turbulence and mixing. However, they cannot simulate 
adequately the most important physical processes involved in climatic change. 
Improvement of our confidence in the ability of climate models to assess 
the climatic impacts of increased CO 2 will require development of model 
validation methods, including determination of the models' statistical prop- 
erties; assembly of standardized data for validation; development of obser- 
vations to validate representations of physical processes; standardization of 
sensitivity tests; development of physical-dynamical and phenomenological 
diagnostic techniques focusing on changes specifically attributable to increased 
C0 2 ; and use of information from planetary atmospheres, laboratory exper- 
iments, and especially contemporary and past climates (see below). 



PREDICTIONS AND SCENARIOS 

A primary objective of climate-model development is to enable prediction of 
the response of the climate system to internal or external changes such as 
increases in atmospheric C0 2 . Predictions consist of estimates of the 
probability of future climatic conditions and unavoidably involve many 
uncertainties. Model-derived estimates of globally averaged temperature 
changes, and perhaps changes averaged along latitude circles, appear to 
have some predictive reliability for a prescribed CO 2 perturbation. On the 
other hand, estimates with greater detail and including other important 
variables, e.g., windiness, soil moisture, cloudiness, solar insolation, are not 
yet sufficiently reliable. Nevertheless, internally consistent and detailed 
specifications of hypothetical climatic conditions over space and time 
"scenarios" may be quite useful research tools for analysis of social 
responses and sensitivities to climatic changes. 



INFERENCES FROM CLIMATE MODELS 

While present models are not sufficiently realistic to provide reliable 
predictions in the detail desired for assessment of most impacts, they can 
still suggest scales and ranges of temporal and spatial variations that can be 
incorporated into scenarios of possible climatic change. 

Mathematical models of climate of a wide range of complexity have been 
used to estimate changes in the equilibrium climate that would result from 
an increase in atmospheric CO 2 . The main statistically significant conclusions 
of these studies may be summarized as follows: 

1. The 1979 Charney report estimated the equilibrium global surface 
warming from a doubling ofCO 2 to be "near 3C with a probable error of 



276 



Summary of Conclusions and Recommendations 7 

1.5C." No substantial revision of this conclusion is warranted at this 
time. 

2. Both radiative-convective and general-circulation models indicate a 
cooling of the stratosphere with relatively small latitudinal variation. 

3 . The global-mean rates of both evaporation and precipitation are projected 
to increase, 

4. Increases in surface air temperature would vary significantly with 
latitude and over the seasons: 

(a) Warming would be 2-3 times as great over the polar regions as over 
the tropics; wanning would be significantly greater over the Arctic than over 
the Antarctic. 

(b) Temperature increases would have large seasonal variations over the 
Arctic, with minimum warming in summer and maximum warming in winter. 
In lower latitudes (equatorward of 45 latitude) the warming has smaller 
seasonal variation. 

5. Some qualitative inferences on hydrological changes averaged around 
latitude circles may be drawn from model simulations: 

(a) Annual-mean runoff increases over polar and surrounding regions. 

(b) Snowmelt arrives earlier and snowfall begins later. 

(c) Summer soil moisture decreases in middle and high latitudes of the 
northern hemisphere. 

(d) The coverage and thickness of sea ice over the Arctic and circum- 
Antarctic oceans decrease. 

Improvement in the quality and resolution of geographical estimates of 
climatic change will require increased computational resolution in the 
mathematical models employed, improvement in the representation of the 
multitude of participating physical processes, better understanding of airflow 
over and around mountains, and extended time integration of climate models. 
It is clear, however, that local climate has a much larger temporal variability 
than climate averaged along latitude circles or over the globe. 



OBSERVATIONAL STUDIES OF CONTEMPORARY AND PAST 
CLIMATES 

Observational studies play an important role in three areas: ( 1) the formulation 
of ideas and models of how climate operates, (2) the general validation of 
theories and models, and (3) the construction of climate scenarios. 

Studies based on contemporary climatic data have provided a useful starting 
point for diagnosis of climatic processes that may prove to be relevant to the 
CO 2 problem. The results of the Global Weather Experiment are now being 



277 



8 CARBON DIOXIDE AND CLIMATE: A SECOND ASSESSMENT 

analyzed and will provide a unique data base for model calibration and 
validation studies. Further analyses and diagnostic studies based on contem- 
porary climatic data sets, particularly the Global Weather Experiment data 
set, should be encouraged. However, scenarios based on contemporary data 
sets do not yet provide a firm basis for climatic assessment of possible CO 2 - 
induced climatic changes, nor should they be considered adequate at present 
for validation of CO 2 sensitivity studies with climate models. 

Studies of past climatic data are leading to important advances in climate 
theory. For example, the large climatic changes between glacial and inter- 
glacial periods are being linked with relatively small changes in solar radiation 
due to variations in the Earth's orbit. If confirmed, these studies will improve 
our understanding of the sensitivity of climate to small changes in the Earth's 
radiation budget. A large multidisciplinary effort will be required to acquire 
the requisite data and carry out the analysis, and such work should be 
encouraged. Studies of past climate are also potentially valuable because 
they deal with large changes of the climate system, including the atmosphere, 
oceans, and cryosphere; because they can reveal regional patterns of climate 
change; and because there is knowledge of the changes in forcing (now 
including changes both in atmospheric CO 2 concentrations and in solar 
radiation) that are apparently driving the system. 



4.1.2 Epilogue 

Despite the relatively brief interval between the Charney report and 
our Panel's study , a considerable volume of additional work had been 
carried out. The virtually exponential growth in scientific research 
in the C0 2 /climate area reflects increasing consciousness of the 
issue's social and scientific importance, burgeoning interest in the 
more general problems of climate/ the development of an active 



278 

community of scientists and institutions concerned with the problem, 
and by no means least continued strong support by funding agencies, 
notably the U.S. Department of Energy. The excellent reviews assembled 
by Clark (1982) and Reck and Hummel (1982) and conducted by the U.S. 
Department of Energy (1983) make a further detailed compendium of 
research unnecessary at this time. Their content, however, demon- 
strates clearly the health of this area of research and the need for 
periodic critical reviews. The consciously conservative assessments 
conducted by National Research Council groups revealed continued 
progress in basic understanding of the climate system. However, our 
understanding of the local and regional details of man-made climate 
changes is advancing only slowly. This frustratingly slow advance 
reflects no lack of talent, effort, or resources but rather the inherent 
difficulty of the task. Although our pace is slow, it is forward and 
provides us with increasingly clear views of future climate. 

Our Panel considered a scenario of increasing C0 2 concentrations 
generally consistent with that postulated by an international assess- 
ment group in 1980, i.e., a slow growth leading to a concentration of 
about 450 ppm by about 2025. This scenario lies within the range of 
uncertainty suggested by the later studies of Nordhaus and Yohe (this 
volume, Chapter 2, Section 2.1) and Machta (this volume, Chapter 3, 
Section 3.5) presented in this report and is consistent with their 
estimates of a doubling of airborne CC>2 toward the end of the twenty- 
first century. As Machta describes below in Section 4.2, the growing 
concentrations of other radiatively active trace gases considerably 
complicate the problem. Tropospheric ozone, methane, nitrous oxide, 
and the chlorofluoromethanes also interact with thermal radiation and 
can produce significant additional perturbations to the Earth's heat 
budget. Thorough discussions have been presented by Chamberlain et al. 
(1982) , Ramanathan (1982) , and Hansen et al. (1982) . While projection 
of the future concentrations of these gases is even more problematical 
than for C0 2 , it does not seem improbable that the perturbation of 
the heat budget due to these trace gases could approach the magnitude 
of the perturbation due to C0 2 alone. If a doubling of atmospheric 
concentration of C0 2 is attained in the latter part of the next cen- 
tury, a concomitant rise in concentrations of other greenhouse gases 
would imply that the climatic equivalent of a doubling would be reached 
much sooner. 

As Revelle and Suess (1957) observed, by changing the atmosphere's 
composition mankind is conducting a great and unprecedented geophysical 
experiment. Since we have no laboratory analog of this experiment, we 
must attempt to predict its outcome by recourse to some model natural 
or analytical. In view of the complexity of the global climate system, 
with its myriad possibilities for unexpected and counterintuitive feed- 
backs and responses, the most desirable model would be the Earth itself. 
Indeed, Budyko and Yefimova (1981) attempted to relate paleoclimatic 
reconstructions to estimated contemporary CO 2 concentrations and have 
reached conclusions consistent with those drawn from numerical models. 
Others (e.g., Kellogg, 1977) have cited previous warm periods in the 
Earth's history as possible guides to the regional pattern of climatic 
changes in a CO 2 -warmed Earth. Both approaches are hampered by 



279 



inadequacies in data particularly in chronoloqv 
causality. For example, -' --- ' 



paced by orbital changes, and in these cases CO ^ ^ 
responses to rather than instigators of the 2 *** *** ** 
Similarly, the warm climate of the e< 
model of a warmer Earth, has been pla 
Otto-Bliesner, 1982) in terms of changes' in 
close study shows a complex time sequence of 

to CO-: 



in 
seem to be 



changes . 

and 

although 
Thus the 



In the absence of satisfactory natural models, we must turn to 
mathematical models based on the most reliable physical and empirical 
relationships that we can muster. For example, Idso (1980, 1982) 
employed an empirically calibrated linear model relating radiant energy 
absorbed at the Earth's surface and surface air temperature, but our 
Panel found his analysis incomplete and misleading. More complete 
models treat the entire atmospheric column and calculate numerically 
the exchanges of sensible, latent, and radiative energy between layers 
and with space. Extension to two and three dimensions permits the 
energy balance and climate of the globe to be simulated with consid- 
erable realism and allows for interaction of climate with the oceans 
and with changes in surface characteristics and snow/ice cover. 

The results of a number of model simulations of contemporary climate 
and the climate corresponding to increased C0 2 were reviewed by our 
Panel. Further comprehensive reviews have been made by Schlesinger 
(1983a,b), Reck (1982), and Budyko et al. (1982). In common with our 
Panel, these generally take as a convenient index the calculated 
equilibrium change in globally averaged surface air temperature for 
doubled (or sometimes quadrupled) C0 2 . The values from studies with 
comprehensive models and realistic boundary conditions lie within the 
range suggested by the Charney Panel in 1979 and which our Panel found 
no grounds for changing. In this connection, one must recall that this 
range represents the best judgments of the two panels based on a small 
sample of inhomogeneous but because of common physical assumptions 
not independent numerical experiments. Continued research will, I hope, 
result in a sharpening of these estimates. Two recent simulations with 
comprehensive general-circulation models at the National Center for 
Atmospheric Research (Washington and Meehl, 1983) and the NASA Goddard 
Institute for Space Studies (Hansen, 1983) have yielded results also 
within or very near this range. 

The global mean temperature index permits us to compare in general 
terms the potential magnitude and rate of climatic changes due to CO 2 
with natural changes of the past. As suggested above, global mean 
surface temperature in the later decades of the next century may be 1.5 
to 4.5c warmer than today. At the lower end of this range, the change 
is comparable in magnitude with the difference between the cold decades 
of "the Little Ice Age" or the early Holocene warm period and today. 
The higher end approaches in magnitude the difference between the last 
glacial maximum and today and enters a climatic range with which mankind 
has had little experience. Rates of change due to C0 2 are projected 
to be a few tenths of a degree Celsius per decade. As discussed by the 



H 



* f 

I 

5 

!) 



280 

Ad Hoc Panel on the Present Interglacial (1974) and by Clark et al. 
(1982), such rates are for short periods comparable with the rapid 
changes observed in some regions in the earlier part of this century or 
at the onset of the Little Ice Age. If sustained for a century or 
more, the most recent parallel is with the retreat of the Wisconsin 
glaciation. 

While convenient and reasonably unambiguous, estimates of projected 
changes in equilibrium global mean temperature are not very useful. 
Parameters of climate other than temperature precipitation, storm 
tracks, cloud cover, for example and the frequencies of extreme events 
are at least as important in determining the real impact of climatic 
change. For example, tropical storm formation seems to be related to 
sea-surface temperature. Although hurricanes are not explicitly treated 
at present in general-circulation models, one might infer that warmer 
ocean temperatures would increase their penetration into midlatitudes; 
such inferences and the topic of climatic extremes deserve careful 
investigation. In general, the impacts of changing climate would be 
felt most acutely in terms of local changes that can be expected to 
vary widely across the Earth's surface. Moreover, as our Panel noted, 
the pace of change will be slowed and its evolution complicated by the 
ocean's buffering effects. Here recent measurements of transient 
tracers in the ocean (cf. Brewer, this volume. Chapter 3, Section 3.2) 
have tended to confirm the notions of the ocean's circulation and 
thermal capacity expressed in our report. Thus, understanding of the 
transient and local responses of climate to increasing CC>2 is far 
more relevant to our concerns than the ultimate globally averaged 
change in equilibrium temperature. 

Despite the general agreement on the overall magnitude of the C02 
effect, our understanding of these regional and temporal effects is only 
rudimentary. The Panel identified a few changes in zonally averaged 
quantities that appeared to have some statistical reliability. However, 
as noted by Manabe et al. (1981) and exhibited in the comparisons of 
Schlesinger (1982) , more detailed local and regional responses are 
smothered in a sea of noise. Assessment of the truly relevant aspects 
of man-made climatic change has only just begun. 

Climate changes over the United States and over other major agricul- 
tural regions of the world are naturally of great interest. Unfortu- 
nately, there is at present little that can be responsibly asserted 
about the details of such changes. In the Panel's report, we pointed 
out a few relevant inferences that seemed to be emerging from some 
experiments, notably a tendency toward summer dryness in midlatitudes 
(e.g., Manabe et al. r 1981; Hayashi, 1982). These suggest, for example, 
some expansion of steppe and desert climates in the latitudes of the 
United States with increased C0 2 . This inference is consistent with 
paleoclimatic data on warmer periods, although as noted above the 
analogies are by no means precise. As we gain more confidence in the 
regional details of climate simulations, analyses of climate model 
results in terms of climate categorizations tuned to the needs of impact 
analysis will become useful. 

Climate models must be markedly improved and much more analysis of 
the implications of their results must be done before we will be able 



281 

to place useful confidence in their detailed results. The most chal- 
lenging, and perhaps the most intractable, problem is cloudiness. It 
is easy to show that small changes in cloudiness can alter the Earth's 
heat budget as much or more than the expected changes in CO 2 concen- 
trations. Models with different formulations for cloudiness show great 
differences in global and regional climate sensitivity, even if their 
simulations of contemporary climate and of globally averaged changes 
are comparable. Our Panel concluded that "One should not trust model 
prediction schemes until they produce meaningful simulations of observed 
seasonal cloud cover and the seasonal radiation components." I believe 
that this reservation still stands and presents the outstanding unsolved 
problem in climate modeling. The par ameterizat ions of boundary-layer 
convective processes particularly in the tropics and radiation trans- 
port also embody significant uncertainties (Kandel, 1981; Luther, 1982). 

Despite our reservations about climate models we have no choice but 
to use them if we wish to assess the possibilities for changed climate 
in a changed atmosphere. We can shore up our confidence by conscien- 
tiously validating models through comparison with nature. The three- 
dimensional general-circulation models, in particular, can be closely 
compared with the real world. Indeed, as summarized for example by 
Gates (1982) , the models simulate reasonably well the principal 
features of today's mean climate, the annual march of the seasons, and 
even the markedly different climates of the distant past. Of course, a 
most reassuring validation would be the unequivocal detection of the 
C02-induced climate changes that the models predict to be currently 
talcing place. This problem is discussed at length in Chapter 5 by 
Weller et al., who also conclude from empirical studies that the real 
sensitivity of the climate system to C0 2 increases is in the lower 
part of the range indicated by models. 

Quantitative estimates of the uncertainties in model predictions 
would be extremely useful. Some crude notion may be gained from 
studying the range of results obtained by different investigators 
employing different models and methodologies. Indeed, such results are 
the primary source for the uncertainty estimates proposed in the Charney 
report and left unchanged by our Panel. Attempts at more rigorous 
analysis are also being made (Hall et al., 1982). However, it must be 
recognized that all modelers incorporate similar ensembles of variables 
and physical processes and employ fundamentally similar algorithms 
(Schneider and Dickinson, 1974) . One must always admit the possibility 
of some overlooked or underestimated feedback, e.g., cloudiness, that 
would markedly change the results. Careful probabilistic analysis of 
model simulations (e.g., Hayashi, 1982) can more clearly separate 
statistically significant conclusions from meaningless noise; the 
conclusions are usually discouraging, but the sparse scraps of sig- 
nificance that remain become even more precious. Finally, one may 
attempt to delimit the uncertainties attributable to various possible 
sources of sensitivity through numerical experimentation. Unsuspected 
and possibly larger sources of error may lurk in the wings. Neverthe- 
less, we can hardly expect policymakers to lend credence to our 
predictions of climatic changes until we can demonstrate that changes 
are actually taking place (see Chapter 5) and to some degree quantify 



r 



282 

the credibility of our forecasts. Thus, the development of objective 
confidence limits for climate sensitivity estimates (e.g., Katz, 1982) 
is an important task for climate modelers. 

Our Panel discussed at length some dissenting inferences of the 
magnitude of C02 f s effect on climate, I believe that our report 
fairly assessed these assertions and put them in proper perspective 
with respect to other research. 

In summary f the conclusions of our study appear to remain valid. 
Man-made increases of C02 and other trace gases in the atmosphere may 
be reasonably expected to change climate significantly within the 
lifetimes of a large fraction of the world's inhabitants who are alive 
today. (According to Ausubel and Stoto, 1981, 40% of the current 
population will still be alive 50 years hence.) The change will be 
large and rapid; it will be greater in global terms than any natural 
climate changes that civilized man has yet experienced, although, as 
Schelling observes in Chapter 9 of this report, far less than the 
climate changes mankind has voluntarily undertaken through migration. 
We have some general notions of how the climate change will be dis- 
tributed across the face of the Earth. In particular, there are 
indications of dryer and hotter summers for some already overheated and 
underwater ed regions of our country, but these are as yet a very 
uncertain basis for decision making. There is a good prospect that 
further research can slowly sharpen and validate our predictive tools 

111 to give us more useful answers (see C02/Climate Review Panel, 1982, 

pp. 48-50) . 

References 

Ad Hoc Panel on the Present Interglacial (1974) . Report. Interdepart- 
mental Committee for Atmospheric Sciences, Federal Council for 
Science and Technology. ICAS 18B-PY75. Washington, D.C. 

Ausubel, J. H., and M. A. Stoto (1981). A Note on the Population Fifty 
Years Hence. International Institute for Applied Systems Analysis, 
Laxenburg, Austria. 

Budyko, M. I., and N. A. Yefimova (1981). Impact of carbon dioxide on 
climate. Meteorol. Hydrol. 2; 5-17. 

Budyko, M. I., K. Ya. Vinnikov, and N. A. Yefimova (1982). The 
dependence of the air temperature and precipitation on carbon 
dioxide concentration in the atmosphere. Meteorol. Hydrol. 4; 5-13. 

Chamberlain, J. W. , M. M. Foley, G. J. MacDonald, and M. A. Ruderman 
(1982) . Climatic effect of trace constituents. In Carbon Dioxide 
Review; 1982 . W. C. Clark, ed. Oxford U. Press, New York. 

Clark, W. C., ed. (1982). Carbon Dioxide Review: 1982. Oxford U. 
Press, New York, 469 pp. 

Clark, W. C., K. H. Cook, G. Marland, A. M. Weinberg, R. M. Rotty, P. 
R. Bell, L. J. Allison, and C. L. Cooper (1982) . The carbon dioxide 
question: a perspective for 1982. In W. C. Clark, ed. (1982), pp. 
3-43. 

Climate Research Board (1979) . Carbon Dioxide and Climate; A 

Scientific Assessment. National Academy of Sciences, Washington, 
D.C. 



283 

C02/Climate Review Panel (1982) . Carbon Dioxide and Climate; A 

Second Assessment, National Research Council, National Academy 

Press, 72 pp. 
Gates, W. L. (1982). Paleoclimatic modeling a review with reference 

to problems and prospects for the pre-Pleistocene. In Climate in 

Earth History. Geophysics Study Committee, National Academy of 

Sciences, Washington, D.C., pp. 26-41. 
Geophysical Fluid Dynamics Laboratory (1982) . Activities FY80-Plans 

FY81. Environmental Research Laboratories, National Oceanic and 

Atmospheric Administration. 
Hall, M. C. G., D. G. Cauci, and M. E. Schlesinger (1982). Sensitivity 

analysis of a radiative-convective model by the adjoint method. J. 

Atmos. Sci. 39:2038-2050. 
Hansen, J. E., A. Lacis, and S. A. Lebedeff (1982). Commentary. In W. 

C. Clark, ed. (1982), pp. 284-289. 
Hansen, J. E. (1983) . Climate model sensitivities to changed solar 

irradiance and C02- In Climate Processes and Climate Sensitivity, 

Maurice Ewing Series, Vol. 4. American Geophysical Union, 

Washington, D.C. 
Hayashi, Y. (1982). Confidence intervals of a climatic signal. . 

Atmos. Sci. 39:1895-1905. 
Idso, S. B. (1980). The clima to logical significance of a doubling of 

the earth's atmospheric carbon dioxide concentration. Science 

207:1462-1463. 
Idso, S. B. (1982a) . A surface air temperature response function for 

earth's atmosphere. Boundary Layer Meteorol. 22:227-232. 
Idso, S. B. (1982b). Carbon Dioxide: Friend or Foe? IBR Press, 

Tempe, Ariz., 92 pp. 
Kandel, R. S. (1981). Surface temperature sensitivity to increased 

atmospheric C0 2 . Nature 293:634-636. 
Katz, R. W. (1982). Statistical evaluation of climate experiments with 

general circulation models: a parametric time series modeling 

approach . J. Atmos. Sci. 39; 1446-1455 . 
Kellogg, W. W. (1977). Effects of Human Activities on Global Climate. 

Tech. Note No. 156 (WMO No. 486) . World Meteorological Organization, 

Geneva, 47 pp. 
Kutzbach, J. E., and B. L. Otto-Bliesner (1982). The sensitivity of 

African-Asian Monsoon climate to orbital parameter changes for 9000 

years BP in a low-resolution general circulation model. J. Atmos. 

Sci. 39; 1177-1188 . 
Luther, F. M. (1982). Radiative effects of a C02 increase: results 

of a model comparison. In Proceedings; Carbon Dioxide Research 

Conference; Carbon Dioxide, Science and Consensus* U.S. Dept. of 

Energy, CONF-820970, III-177-III-193. 
Manabe, S., R. T. Wetherald, and R. S. Stouffer (1981). Summer dryness 

due to an increase of atmospheric C02 concentration. Climatic 

Change 3; 347-386. 
Ramanathan, V. (1982). Commentary. In W. C. Clark, ed. (1982), pp. 

278-283. 
Reck, R. A. (1982) . Introduction to the Proceedings of the Workshop. 

In Reck and Hummel (1982) . 



284 

Reck, R. A., and V. R. Hummel (1982). Interpretation of Climate and 
Photochemical Models, Ozone and Temperature Measurements. AIP 
Conference Proceedings No. 82. American Institute of Physics, New 
York, 308 pp. 

Revelle, R. , and H. E. Suess (1957). Carbon dioxide exchange between 
atmosphere and ocean and the question of an increase of atmospheric 
C0 2 during the past decades. Tellus 9:18-27. 

Schlesinger, M. E. (1983a) . Simulating CO 2 -induced climatic change 
with mathematical climate models: capabilities, limitations, and 
prospects. In Proceedings, Conference on Carbon Dioxide, Climate, 
j and Consensus, Coolfont, Virginia. U.S. Dept. of Energy, Washington, 

i D.C. 

! Schlesinger, M. E. (1983b) . A review of climate models and their 

I simulation of C0 2 -induced warming. Intern. J. Environ. Studies 

j 20.: 103-114. 

i Schneider, S. H., and R. E. Dickinson (1974). Climate modeling. Rev. 

\\ Geophys. and Space Phys. 12:447-493. 

I U.S. Department of Energy (1983). Proceedings; Carbon Dioxide Research 

> | Conference; Carbon Dioxide, Science, and Consensus. CONF-8 20 9 7 , 

l\ February 1983. 

!; Washington, W. M., and G. A. Meehl (1982). A summary of recent NCAR 

[ general circulation experiments on climatic effects of doubled and 

s; quadrupled amounts of C0 2 . In Proceedings, Conference on Carbon 

j| Dioxide, Climate, and Consensus, Coolfont, Virginia. U.S. Dept. of 

;] Energy, Washington, D.C. 

Washington, W. M., and G. A. Meehl (1983). General circulation model 
experiments on the climatic effects due to a doubling and 
quadrupling of carbon dioxide concentrations. J. Geophys. Res., in 
press. 



285 

4.2 EFFECTS OF NON-C0 2 GREENHOUSE GASES 
Lester Machta 



C0 2 is not the only gas or geophysical property capable of modifying 
the future climate. While a discussion of every potential climate 
modifier lies beyond this report, certain of them offer sufficient 
similarities to CO 2 (e.g., they are also "greenhouse" gases) that a 
brief survey is justified. Non-CO 2 greenhouse gases (and other 
climate modifiers) bear on the CO 2 issue in several important ways: 
(1) they can enhance the climate changes expected from rising atmo- 
spheric C0 2 and hence confuse the expected CO 2 changes; (2) the 
past climate responses to other forcing functions can aid in the 
interpretation of C0 2 warming; but (3) perhaps most important/ 
predictions of the future climate require that all potential factors be 
taken into account. 

In the past few decades the remarkable increase in interest in 
atmospheric chemistry along with improved technology have made it 
possible to measure changes with time of the concentration of many 
constituents of air not formerly possible. Despite this new 
capability, there may still be other greenhouse gases beyond those 
noted below that are not yet measured. 

Chlor of luorocar bons . This class of gases originates from industrial 
activities and has been emitted to the atmosphere during the past 50 
years. They are increasing in the atmosphere approximately as expected 
from their growth in emissions. CFO11, CFC-12, and CFC-22, the three 
most abundant ones, all have long residence times in the air (tens of 
years) so that they can accumulate. Figure 4.1 illustrates typical 
time histories of CFC-11 and CFC-12 at ground level. Note that, like 
C02, with its fairly long atmospheric residence time, the concentra- 
tion in air will usually increase even if the rate of emissions 
decreases; this is the case for CFC-11 and CFC-12 during the past few 
years. Both the sources and sinks of the chlorof luorocar bons are 
believed to be known. The emissions from industrial production and 
produce uses (such as aerosol propellants) , for which good estimates 
are published by the Chemical Manufacturers Association, represent the 
only source of any consequence. Photochemical destruction, mainly in 
the stratosphere, and very slow uptake by the oceans are the only known 
significant sinks. Theoretically, chlorof luorocarbons are implicated 
as potential destroyers of stratospheric ozone, which in turn could 
result in human health and ecological damage from increased ultraviolet 
radiation. Since some consideration has been given to restricting 
their emissions, an extrapolation of current or past growth rates to 
predict future atmospheric concentrations may be unwise at this time. 

Nitrous oxide, it is likely that most nitrous oxide in the air has 
come from denitrif ication in the natural or cultivated biosphere. One 
would therefore expect to find the largest part of atmospheric nitrous 
oxide to be derived from nature, unrelated to human activity. Recent, 
careful measurements by a few investigators (Weiss, 1981; U.S. Govern- 
ment, 1982) have suggested a small growth rate of the concentration of 
nitrous oxide in ground level air at remote locations as illustrated in 



286 



380 
340 
300 
260 
220 



1 - 1 

CFC-11 



.* 





11.3 PPT/YR 



CFC ' 12 



N!O 



15.7 PPT/YR 

H 1 



1.0 PPB/YR 
J - 1 



220 
200 
180 
160 
140 



310 



300 



290 



1977 1978 1979 1980 1981 1982 



FIGURE 4.1 Upper panels: Measurements of chlorofluorocarbon (CFC-11, 
CFC-12) concentration in the atmosphere/ Mauna Loa Observatory. Bottom 
panel: Measurements of atmospheric concentration of nitrous oxide in 
ground- level air at remote locations. [Source: Geophysical Monitoring 
for Climate Change (GMCC) f Air Resources Laboratory, Rockville, Md.] 



Figure 4.1. The source of the small increase is unknown/ but a prime 
candidate is the continued expanded use of nitrogen fertilizers around 
the world to improve agricultural productivity. If so, the current 
slow increase is likely to continue into the foreseeable future since 
food demands will grow with population size. It has been suggested 
that since nitrous oxide is stable in the troposphere and is implicated 
in potential ozone destruction, there might be a move to try to restrict 
fertilizer usage. Figure 4.1 suggests, however, that the rate of 
increase of nitrous oxide in ground level air is so small, perhaps 
0.25% per year, that many decades would pass before an increase of 
nitrous oxide would raise concern for ozone depletion. 

Methane. The most abundant hydrocarbon, often called natural gas, 
is increasing in the atmosphere. It is thought to be a natural con- 
stituent of the air arising as it does from many biological processes 
and perhaps seeping out of the Earth. Measurements in the 1950s and 
1960s had large error bars, and there were spatial differences so that 
the observed temporal variability was not viewed as an upward trend. 
However, in the late 1970s several investigators using gas chromatog- 



287 




1965 



1970 



YEAR 



1975 



1980 



FIGURE 4.2 Northern hemisphere methane measurements/ compiled from 
various sources, from 1965 to 1980 (log scale). The straight-line 
exponential best fit corresponds to an annual increase of about 1.7% 
per year (Rasmussen et al., 1981). 



raphy have unequivocably demonstrated an upward trend. Rasmussen and 
Khalil (1981) have combined their recent measurements with earlier ones 
to suggest that the trend existed since at least 1965, as shown in 
Figure 4.2. Craig and Chow (1982) have found, from measurements of 
methane in ice cores, that concentrations prior to the sixteenth 
century were 0.7 ppmv. 

Rasmussen and Khalil suggest that the expansion in the number of 
farm animals and rice production might well explain, at least quali- 
tatively, the atmospheric methane growth. Other biological activities 
such as termite destruction of wood (Zimmerman et al., 1982) and 
possible leakage from man's mining and use of fossil methane might also 
contribute to methane in the air; their contribution to a growth of 
methane in air is less clear. The higher concentrations far north of 
the equatorial region suggest the termite source to be minor. The 
relatively rapid recent increase with time, about as fast as for C0 2 , 
combined with the uncertainty as to its origin are both intriguing 
features of the methane growth in air. It is possible that the trend 
in the stable i so topic content of the carbon in methane might shed 
light on the source of the growth. Thus, atmospheric methane is now 
about -40%, while that derived from biological activity is about -60%. 



288 

If the growth in atmospheric methane is due to increased biological 
sources, the carbon in methane should become more negative with time. 
The interpretation of such isotopic trends will require an understanding 
of appropriate fractionation factors as the methane moves from one 
reservoir to another. 

There is no reason to expect the upward trend in atmospheric methane 
concentration to stop soon since the most likely sources of methane are 
related to population size. In the long run f those sources that are 
dependent on the size of a biospheric feature (e.g., cows or rice 
paddies) will ultimately be limited by space. Thus, the growth rate in 
atmospheric concentration illustrated in Figure 4.2 might continue for 
many decades but probably not for centuries. A better understanding of 
the source of the upward trend would improve this prediction. 

Tropospheric ozone. Tropospheric ozone was originally believed to 
be primarily a consequence of transport from stratospheric ozone by air 
motions. It can also be created within the troposphere by man and 
nature. Locally, as in the Los Angeles Basin, large amounts of ozone 
are derived from oxides of nitrogen, hydrocarbons, and sunlight. Few 
scientists believe that these local sources of pollution can increase 
the upper-tropospheric concentrations of ozone since ozone is so 
reactive that its lifetime in the lower atmosphere is relatively short, 
no more than a few days. Nevertheless, an analysis of a limited number 
of measurements of ozone in the 2- to 8-km layer in the northern 
hemisphere suggest an upward trend as evidenced in Figure 4.3 from 
Angell (1983). Note that no trend can be found in southern Australia. 
It has been suggested by Liu et al. (1980) that this increase of mid- 
and upper-troposphere ozone concentration of the northern hemisphere 
results from photochemical reactions of the oxides of nitrogen and 
hydrocarbons emitted by high-flying jet aircraft. Since the lifetime 
of an ozone molecule in the upper troposphere is also relatively short, 
little accumulation takes place. An increase in concentration must 
therefore reflect a continual increase in aircraft emissions, if that 
is the source. During most of the period illustrated in Figure 4.3, 
the number of jet aircraft as increasing in the northern hemisphere. 

Some other gases. The ALE program of the Chemical Manufacturers 
Association and CSIRO have measured several other gases at the 
Australian Baseline station in Tasmania. At least two of three gases 
(in addition to some already noted above) show upward trends and may 
have absorption lines in the infrared window of -the electromagnetic 
spectrum, making them potential greenhouse gases: carbon tetrachloride 
(CC 14 ) and methyl chloroform (C^CCl^) . The growth of carbon tetra- 
chloride reported from Tasmania is about 1% per year since 1976, but 
the methyl chloroform is closer to 10% per year since 1979. Very likely 
both of these gases possess both natural and man-made sources. On the 
other hand, measurements at the Mauna Loa Observatory exhibit no or 
insignificant increases in carbon monoxide (CO) . It is likely that the 
list of atmospheric gases studied for their trends and potential green- 
house effects will grow in years to come: the study of greenhouse 
gases other than C0 2 is still in its infancy. 

The atmospheric concentrations of these trace gases are not all inde- 
pendent of one another. Complicated chemical reactions among these 



o> 

i 



LJU 



LLJ 



LLI 
Q 



20 

10 



10 



o 

SL -10 



10 


-10 
-20 



289 

Ozone, 2-8 km 



T 



Resolute 
75 




1965 



1970 



1975 



1980 



YEAR 



FIGURE 4.3 Ozone measurements in the 2- to 8-km layer at various latitudes. 
(Source: Angell f 1983.) 



gases, as well as with other gases not particularly radiatively active/ 
can affect their concentrations. For instance, it has been argued (WMO, 
1983) that increases in carbon monoxide (CO) in the presence of NO can, 
by OH oxidation, produce an increase in 03 and methane (CH^ . 

In addition to chemical reactions with today's atmospheric composi- 
tion, there would likely be new climate-chemistry interactions in the 
future. As the composition changes, the expected higher atmospheric 
water-vapor content will affect the atmospheric chemistry. An increase 
in OH accompanying an increase in HjO could reduce the 03 and CH 4 
otherwise present. 

Thus, unlike C0 2 , which generally does not undergo chemical changes 
in the air, these trace gases frequently do. Not only can the mean 
concentration be affected by other chemicals and sunlight, but distribu- 
tion particularly in the vertical can be influenced (ozone is a prime 
example) . To estimate future concentrations will require more than 
estimates of natural and man-made emission rates, fundamental though 
those rates will be. Increased global coverage of the measurements of 
these gases will also help in separating natural from man-made sources 
of some of these gases. 

Climate effect. Most of the estimates of the climate effects of the 
trace species have been based on 1-D radiative-convective models. 
Typically the calculation involves doubling a reference concentration 



290 

of the gas (for the chlorofluorocarbons, increases from to 1 or 2 ppb 
are used) while other constituents are held constant. Table 4.1 gives 
some estimates of the change in surface temperature due to either a 
doubling of their concentration or an increase from to 1 ppb for the 
halocarbons. The table was adapted from Table 2a in WMO (1983). There 
are other published values, but they generally do not disagree by more 
than about ^30% with the figures given here. 

The models used to obtain these results generally gave a sensitivity 
to doubled CC>2 between 2 and 3C. Thus/ none of the changes of 
individual trace gases approaches C0 2 by itself, but it is clear that 
the summation of all of these potential changes could be of the same 
magnitude as C02 It is worth noting that because the concentration 
of each of these gases is small enough to be treated as optically thin, 
the temperature effect is linearly proportional to their concentration, 
whereas the CC>2 effect depends logarithmically on the concentration. 

The spectroscopic parameters of several of these gases is not well 
known, and even the band strengths of some have not been measured. The 
spectral transmittance and total band absorptions also need to be 
better determined. These improvements will be needed to develop more 
accurate radiative transfer models. It will also help in answering 
questions about band overlap between constituents and with water 
vapor. Such information is needed for better parameterization in 
climate models. 



TABLE 4.1 Some Estimates of Surface Temperature Change due to Changes 
in Atmospheric Constituents Other Than CC>2 



Constituent 


Mixing 
Change 
From 


Ratio Surface 
(ppb) Temperature 
To Change (C) source! 


Nitrous oxide (N 2 0) 


300 


600 


0.3-0.4 


1,3 


Methane (CH 4 ) 


1500 


3000 


0.3 


3,4 


CFC-11 (CFC1 3 ) 





1 


0.15 


1,5 


CPC-12 (CF 2 C1 2 ) 





1 


0.13 


1,5 


CFC-22 (CF 2 HC1) 





1 


0.04 


7 


Carbon tetrachloride (CC1 4 ) 





1 


0.14 


1,5 


Carbon tetrafluoride (CF 4 ) 





1 


0.07 


2 


Methyl chloride (CH 3 C1 3 ) 





1 


0.013 


1,5 


Methylene chloride (CH 2 C1 2 ) 





1 


0.05 


1,5 


Chloroform (CHC1 3 ) 





1 


0.1 


1,5 


Methyl chloroform (CH 3 CC1 3 ) 





1 


0.02 


7 


Ethylene (C 2 H 4 ) 


0.2 


0.4 


0.01 


1 


Sulfur dioxide (S0 2 ) 


2 


4 


0.02 


1 


Ammonia (NH 3 ) 


6 


12 


0.09 


1 


Tropospheric ozone (0 3 ) 


F(Lat,ht) 


2 F(Lat,ht) 


0.9 


4,6 


Stratospheric water vapor (H 2 0) 


3000 


6000 


0.6 


1 



^Sources: 1, Wang et al. (1976); 2, Wang et al. (1980); 3, Donner 
and Ramanathan (1980); 4, Hameed et al. (1980); 5, Ramanathan (1975); 
6, Pishman et al. (1979); 7, Hummel and Reck (1981). 



291 
References 

Angell, J. K. (1983) . Global variation in total ozone and layer-mean 

ozone: an update through 1981. Manuscript, Air Resources 

Laboratory, Silver Spring, Md. 
Craig, H., and C. C. Chou (1982). Methane: the record in polar ice 

cores . Geophys. Res. Lett. 9; 1221-1224 . 
Donner, L., and V. Ramanathan (1980). Methane and nitrous oxide: 

their effect on the terrestrial climate. J. Atmos. Sci. 37:119-124. 
Fishman, J. , V. Ramanathan, P. Crutzen, and S. Liu (1979). 

Tropospheric ozone and climate. Nature 282:818-820. 
Hameed, S., R. Cess, and J. Hog an (1980). Response of the global 

climate to changes in atmospheric composition due to fossil fuel 

burning. J. Geophys. Res. 85:7537-7545. 
Hummel, J. R. , and R. A. Reck (1981). The direct thermal effect of 

CHC1F 2 , CH3CC13 and CH2C12 on atmospheric surface temperatures. 

Atmos. Environ. 15:379-382. 
Liu, S. C., D. Kley, M. McFarland, J. Mahlman, and H. Levy, II (1980). 

On the original of tropospheric ozone. J. Geophys. Res. 

8.5:7546-7552. 
Ramanathan, V. (1975) . Greenhouse effect due to chlorofluorocarbons: 

climatic implications. Science 190:50-52. 
Rasmussen, R. A., and M. A. K. Khalil (1981). Atmospheric methane 

(CH4) : trends and seasonal cycles. J. Geophys. Res. 86:9826-9832. 
U.S. Government (1982). Summary report for 1981. Geophysical 

Monitoring for Climatic Change No. 10. 
Wang, W. C., Y. Yung, A. Lacis, T. Mo, and J. Hansen (1976). Greenhouse 

effects due to man-made perturbation of trace gases. Science 

194:685-690. 
Wang, W. C., J. P. Pinto, and Y. Yung (1980). Climatic effects due to 

halogenated compounds in the Earth's atmosphere. J. Atmos. Sci. 

r?:333-338. 
Weiss, R. F. (1981) . The temporal and spatial distribution of 

tropospheric nitrous oxide. J. Geophys. Res. 86:7185-7196* 
WMO (1983) . Report of the Meeting of Experts on Potential Climatic 

Effects of Ozone and Other Minor Trace Gases. Report No. 14. WMO 

Global Ozone Research and Monitoring Project, Geneva, 38 pp. 
Zimmerman, P. R. , J. P. Greenberg, S. 0. Wandiga, and P. J. Crutzen 

(1982). Termites: a potentially large source of atmospheric 

methane, carbon dioxide and molecular hydrogen. Science 218:563-565. 



Detection and Monitoring of 
5 C0 2 -Induced Climate Changes 

Gunter Weller, D. James Baker, Jr., W. Lawrence Gates, Michael C. 
MacCracken, Syukuro Manabe, and Thomas H. VonderHaar 



5.1 SUMMARY 

Two questions are addressed in this chapter: (1) have we already 
detected a climatic change attributable to increasing C02r and (2) 
what observations and analyses can most effectively enable us to detect 
such changes and to monitor their progress? 

The most clearly defined change expected from increasing atmospheric 
C0 2 is a large-scale warming of the Earth's surface and lower atmo- 
sphere. A number of investigators have examined trends in globally or 
hemispherically averaged surface temperature for evidence of C02- 
induced changes. Although differing in detail because of varying data 
sources and analysis methods, the records of large-scale average 
temperatures reconstructed by a number of investigators are in general 
agreement for the period of instrumental records, i.e., about the last 
100 years. Northern hemisphere temperatures increased from the late 
nineteenth century to the 1940s, decreased until the mid-1970s, and 
have apparently increased again in recent years. The mean temperature 
of the 1970s was about 0.5C warmer than that of the 1880s. To the 
extent that one can judge from scanty data, southern hemisphere tem- 
peratures have increased more steadily by about the same amount. Jin 
view of the relatively large and inadequately explained fluctuations 
over the last century, we do not believe that the overall pattern of 
variations in hemispheric or global mean temperature or associated 
changes in other climatic variables yet confirms the occurrence of 
temperature changes attributable to increasing atmospheric CO? 
concentration. 



This chapter was commissioned by and reviewed by the Climate Research 
Committee of the Board on Atmospheric Sciences and Climate at the 
request of the Carbon Dioxide Assessment Committee. The contributions 
of Hugh W. Ellsaesser, Frederick M. Luther, Robert A. Schiffer, David 
E. Thompson, and Donald J. Wuebbles are also gratefully acknowledged. 
John S. Perry and Jesse H. Ausubel provided logistical, administrative, 
and editorial support. 



292 



293 



Other factors than C0 2 ~such as atmospheric tnrMrm- ^ , 
r Ration-also influence climate. Attempts have been LT\ ** 
for these influences on the temperature record ^K mad ^. to am 
sought-for CO 2 signal stand outwore clearly ^nfo ^1^0^ 
inairect sources of historical data are available for th* ' * 
ceding the short period of instrumental records Moreover str at" 
spheric turbidity has been inferred primarily from voicing activity 
ana solar radiance from such phenomena such as sunspots Shi I 
titative reliability of these inferences is unkno^ * ^ 

Despite these difficulties, a number of investigators, employing 
various combinations of data and methodology, have rela^d the global 
or hemispheric mean temperature record with indices of turbidity and 
solar radiance and with estimates of the effect of increasing C0 2 
Although good agreement between modeled and observed variations has 
been obtained in some of these studies, it is clear that enormous 
uncertainties exist. When^attempts are made to account for climatic 
influences of such other factors as volcanic and solar variations, an 
apparent temperature trend consistent with the trend in C0 9 concen-~ 
tr at ions and simulations with climate models becomes more evident. 
However, uncertainties preclude acceptance of such analyses as more 
than suggestive. Nevertheless, the studies done to date have been "most 
helpful in raising questions , suggesting relationships, and identifying 
gaps in data and observations. 

In essence, the problem of detection is to determine the existence 
ana magnitude of a hypothesized C0 2 effect against the background of 
climatic variability, which may be in part due to internal processes in 
the atmosphere and ocean and in part explainable in terms of fluctua- 
tions in external factors. A reasonable approaches to attempt to 
decompose the record of some climatic parameter, e.g., temperature, 
into a hypothesized "natural if value, a perturbation due to CO 2 , and a 
random component. The "natural" value may be taken as a constant 
long-run preindustrial mean or perhaps that mean corrected for variable 
factors such as volcanic and solar activity. The random component can 
probably be treated as "noise-like" but will have to differ substan- 
tially from both "white noise" and first-order autoregressive noise. 
It is clear that the magnitude of the derived C0 2 signal will depend 
markedly on the hypothesis chosen for the underlying climatic trend and 
ttie change in C02 assumed between the imperfectly known preindustrial 
value and the accurately measured current concentrations. The success 
achieved by several workers in explaining the temperature record in 
diverse ways demonstrates the availability of a number of sets of 
hypotheses that can fit the poorly defined historical data and 
estimated preindustrial concentrations. 

The available data on trends in globally or hemispherically averaged 
temperatures over the last century, together with estimates of C0 2 
changes over the period, do not preclude the possibility that slow 
climatic changes due to increasing Atmospheric C0 2 projections might 
already be under way. If the preindustrial CQ 2 concentration was 
near 3QQ ppm , the sensitivity of climate to CO ? (expressed as 



294 

projected temperature increase for a doubling of CO? concentration) 
might be as large as suggested by the upper half of the range of the 
study of the C0 2 /Climate Review Panel (1982) f i.e., up to perhaps 
4.5C; if the preindustrial CO2 concentration was well below 300 
ppm and if other forcing factors did not intervene/ however , the 
sensitivity must be below about 3C to avoid inconsistency with the 
available record, 

If, as expected, the C0 2 signal gradually increases into the 
future, then the likelihood of perceiving it with an appropriate degree 
of statistical significance will increase. Given the inertia created 
by the ocean thermal capacity and the level of natural fluctuations, we 
expect that achieving statistical confirmation of the C0 2 -induced 
contribution to global temperature changes so as to narrow substan- 
tially the range of acceptable model estimates may require an extended 
period. Improvements in climatic monitoring and modeling and in our 
historic data bases for changes in CO2, solar radiance, atmospheric 
turbidity, and other factors may, however, make it possible to account 
for climatic effects with less uncertainty and thus to detect a C0 2 
signal at an earlier time and with greater confidence. Improved 
monitoring of appropriate variables can be of great importance here by 
allowing improvement and more effective validation of models. A 
complicating factor of increasing importance will be the role of rising 
concentrations of greenhouse gases other than C02. While the role of 
these gases in altering climate may have been negligible up to the 
present, their significance is likely to grow, and their effects will 
be difficult to distinguish from those due to C02 

A monitoring strategy should focus on parameters expected to respond 
strongly to changes in C02 (and other greenhouse gases)* and on other 
factors that may influence climate. Candidate parameters may be 
identified, their variability estimated, and their time evolution 
predicted through climate model simulations. Through analysis of past 
data, continued monitoring, and a combination of careful statistical 
analysis and physical reasoning, the effects of CC>2 may eventually be 
discerned. 

Monitoring parameters should include not only data on the C0 2 
forcing and the expected climate system responses but also data on 
other external factors that may influence climate and obscure C0 2 
influences. Climate modeling and monitoring studies already accom- 
plished provide considerable background for the selection of these 
parameters. Since fairly distinct climate changes are only expected to 
become evident over one or more decades, monitoring for both early 
detection and more rapid model improvement should be carried out for an 



*It will be difficult to distinguish between the climatic effects of 
CC>2 and those of other radiatively active trace gases. Their 
expected relative contributions to climatic change will have to be 
inferred from model calculations and precise monitoring of radiation 
fluxes. 



295 

extended period. Parameters may be selected for early emphasis on the 
basis of the following criteria: 

1. Sensitivity. How does the effect exerted on climate by the 
variables or the changes experienced by the variable on decadal time 
scales compare with that associated with corresponding changes in CO2? 

2. Response characteristics. Are changes likely to be rapid enough 
to be detectable in a few decades? 

3- Signal-to-noise ratio. Are the relevant changes sufficiently 
greater than the statistical variability to be measured accurately? 

4 - Past data base. Are data on the past behavior of the variable 
adequate for determining its natural variability? 

5* Spatial coverage and resolution of required measurements. 

6. Required frequency of measurements. 

7. Feasibility of technical systems. Can we make the required 
measurements? 

Initial application of these criteria leads to the list of 
recommended variables for monitoring given below: 



Priority 
First 



Second 



Monitoring 
Causal Factors by 
Measuring Changes in 

C02 concentrations 
Volcanic aerosols 
Solar radiance 



"Greenhouse" gases 
other than C0 2 

Stratospheric and 
tropospheric ozone 



Monitoring 
Climatic Effects by 
Measuring Changes in 

Troposphere/surface 

temperatures (including 

sea temperatures) 
Stratospheric temperatures 
Radiation fluxes at the top 

of the atmosphere 
Precipitable water content 

(and clouds) 

Snow and sea- ice covers 
Polar ice-sheet mass balance 
Sea level 



In the above list, emphasis has been given to parameters that may 
contribute, either directly or through model improvement, to detection 
of C02 effects at the earliest possible time. Over the long run it 
is important to build up a relatively complete data base of the pos- 
sible causes and effects of climate change and the characteristics of 
climate variability, not simply for detection but also to assist in 
research on and calibration of models of the climate system. Once we 
become convinced that climate changes are indeed under way, we will 
seek to predict their future evolution with increasing urgency and with 
increasing emphasis on parameters of societal importance (e.g., sea 



296 

level, rainfall) . We should thus anticipate that a detection program 
will gradually evolve into a more comprehensive geophysical monitoring 
and prediction program. 

It should be emphasized that the strategy proposed here is a single 
tentative step in what must be an iterative process of measurement and 
study. In subsequent steps/ we urge more thorough evaluation with 
greater attention , in particular, to the following; 

(a) Time: When could we have a record long enough to make a 
meaningful contribution to policy formulation? 

(b) Long-run values for model calibration and verification. 

(c) Cost. 

(d) Societal importance. 

Collection of the desired observations will require a healthy global 
observing system, of which satellites will be a major component. Satel- 
lites can provide or contribute to long-term global measurements of 
radiative fluxes, planetary albedo, snow/ice extent, ocean and atmo- 
spheric temperatures, atmospheric water content, polar ice sheet volume, 
aerosols, ozone, and trace atmospheric components; a well-designed and 
stable program of space-based environmental observation is essential if 
we are to monitor the state of our climate. Table 5.1 summarizes 
requirements and technical systems for monitoring high-priority 
variables. 

We will also have to continue to improve our climate models in order 
to reduce the uncertainties in predictions of climate effects and to 
validate the models against observations (although we believe that 
climate models are at present sufficiently sound and detailed to enable 
us to identify a set of variables that could form the basis for an 
initial monitoring strategy) . Also, statistical techniques for asses- 
sing the significance of observed changes will have to be improved so 
as to deal with the characteristics of the monitored variables. In the 
end, however, confidence that we have detected the effect of C0 2 will 
have to rest on a combination of both statistical testing and physical 
reasoning. 

Finally, we must recognize that despite our best efforts there will 
always remain room for differing interpretations of data. Within our 
own country, different investigators have reached quite different con- 
clusions from the same evidence. The detection issue is inherently 
global, and interest is growing throughout the world. It is only to be 
expected that investigators in different countries may also reach 
different conclusions. This diversity of judgments may lead to 
unnecessary confusion and division among nations. There is thus a 
clear need for an international focal point or clearinghouse for data, 
analyses, studies, and periodic assessments relevant to the climatic 
effects of increasing C02- 



297 

5.2 HAVE CO 2 - INDUCED SURFACE TEMPERATURE CHANGES ALREADY OCCURRED?* 
5.2.1 Introduction 

In 1938, G. S. Callendar suggested that mankind's fossil fuel emissions 
were causing an increase in atmospheric CO2 concentration and that 
this, in turn, was leading to the climatic warming that had been 
detected early in this century. With new radiative calculations 
performed by Plass (1953, 1956) and an improvement in understanding of 
oceanic chemistry due to Revelle and Seuss (1957) , the quantitative 
basis for Callendar 's suggestion became more acceptable. In January 
1961, in one of his last papers, Callendar compiled global climatic 
data on temperature trends in order to assess the possible role of 
increasing C0 2 concentrations (Figure 5.1). He found "...that the 
observed distribution of recent climatic trends over the earth is not 
incompatible with the C0 2 hypothesis, and that in certain cases the 
latter [i.e., the C0 2 hypothesis] can supply a reasonable explanation." 
The carefully measured tone of this endorsement of his own hypothesis 
resulted from its failure, in his mind, to explain three troubling 
features of the observed climatic trends. These features included the 
following : 

1. "...the rising temperature trend is very small in the south 
temperate zone as compared to that in the north." 

2. "The tendency for precipitation to decrease in many warm 
regions, and remain sub-normal during the first three or four decades 
of this century (Kraus, 1955)." 

3. "...the big rise of temperature in most parts of the sub-arctic 
zone during the 1920s and 1930s... as compared to the changes in the 
lower latitudes." 

Callendar felt that some aspects of these features could be reconciled 
with the C0 2 theory, by, for example, considering the large thermal 
inertia of the oceans, greatly delayed transport of fossil fuel derived 
C0 2 to the southern hemisphere, radiatively induced stabilization of 
clouds, lower-latitude induced contraction of the polar vortex, and the 
consideration of other factors to explain short-term fluctuations.t 



*Some of the material in this section also appeared in MacCracken 
(1983) . 

tThe thermal inertia of the oceans, and not delayed cross- 
equatorial transport of C0 2/ is now believed to be important in 
delaying the temperature response of the southern hemisphere (Climate 
Research Board, 1979; CO^Climate Review Panel, 1982). Radiatively 
induced stabilization of clouds is not now recognized as the cause of 
geographical variations in CO 2 -induced rainfall changes; rather r the 
shifting circulation pattern, including contraction of the polar 
vortex, leads to these variations. Ice-albedo feedback and the 
presence of a polar temperature inversion are now believed to lead to 
the amplification of temperature changes in polar regions. 



298 







CO 










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Carbon dioxide 
Solar radiance 
Stratospheric aerosols 


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300 



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temperate 

zone 

Tropical 

South 

temperate 

zone 

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1880 



I 



1900 



1920 
YEAR 



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1940 



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60 -73 N 



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temperate 

zone 

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South 

temperate 

zone 



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1860 




1960 



FIGURE 5.1 Reconstructions of changes in surface air temperature for 
different latitude regions by Callendar (1961) applying (top) a 20-year 
moving average and (bottom) a 5-year smoothing to the data. 



301 

He did not, however, believe that volcanic eruptions or solar varia- 
tions could adequately explain the observed long-term climatic warming 
and the latitudinal pattern of the temperature change. 

It is interesting to contemplate how much more forceful Callendar's 
endorsement of his C(>2 hypothesis would have been if he had had 
available the results of today's models. Although present climate 
model simulations indicate a sensitivity about half that found by 
Plass, they do project smaller temperature increases in the southern 
hemisphere (due presumably to ocean inertia and albedo effects) , provide 
for regions of both increasing and decreasing precipitation, and 
indicate that there should indeed be a large polar amplification of the 
C0 2 -induced global temperature increase. Predictions and observa- 
tions would have seemed to be in agreement, and detection might have 
been claimed. 

Perhaps quite fortunately, the absence of present-day model results 
and the lag in theoretical understanding prevented a consensus from 
developing, however, for later in 1961, J. Murray Mitchell, Jr., 
presented his now classic paper on global temperature trends. This 
paper reanalyzed the data on temperatures of the previous hundred years 
and found that the previously claimed warming, when properly weighted 
by area, was not really as large as had been thought by Callendar 
(1961) and Willett (1950) . This finding actually would have improved 
Callendar's agreement with today's model results. More importantly, 
however, Mitchell found that the climate during the 1950s was, in fact, 
cooling (see Figure 5.2) at a time when the CO2 trend should have 
exerted a warming influence.* The reversal in temperature trends found 
by Mitchell set the stage for intensive efforts during the last 20 
years to untangle the web of factors that influence the climate. 

The extensive monitoring, research, and analysis since then has 
taught us several lessons that should enable us to set a course for 
identifying the projected CC^-induced climate response. This chapter 
will review some of the diagnostic studies that have been performed and 
the problems that have arisen in the attempt to detect CO 2 - induced 
surface-air-temperature change. The chapter is not intended to serve 
as a comprehensive review of all studies analyzing the climate of the 
last 100 years but rather attempts to point out general problems and to 
use selected studies to illustrate how difficult these problems are to 
resolve and where we stand now with regard to identifying the climatic 
effects of increasing C02 concentrations. 

5.2.2 Requirements for Identifying CQp-Induced Climate Change 

To achieve widespread confidence in the projected climatic effects of 
increasing CO 2 concentrations, both a consensus of climate model 



*Concentrations were increasing relatively slowly, so very little 
warming should actually have been expected during this period. The 
cooling, however, is deserving of explanation. 



302 



0.8- 
0.7 
0.6 
0.5 



_. 0.4 

o 

~ 

0.3 



0.2 
0.1 

0.0 
-0. 



0-80 N 
0-60 N 
060 N 




1860 



I ..., I " 
1880 1900 1920 

YEAR 



1940 



1960 



FIGURE 5.2 Construction of changes in surface air temperature for 
different latitude bands by Mitchell (1961) . Average data are compiled 
for successive pentads starting with 1870-1874. 



results and unambiguous identification in recent climatic records of 
the initial stages of the projected C0 2 -induced warming are necessary. 
As expressed in recent reports of the National Research Council (Climate 
Research Board, 1979; CO 2 /Climate Review Panel, 1982), the change in 
global average surface temperature (the most closely scrutinized model 
result) due to doubled C0 2 concentration is likely to be between 1.5 
and 4.5C, with more physically comprehensive models giving results in 
the range of 2 to 3.5'C. While increases in the global average temper- 
ature of even 1.5C would lead to a historically unprecedented climatic 
situation, the range of estimated temperature (and other climate) 
changes remains wide and introduces considerable uncertainty into the 
detection debate. As the reasons for the spread in the estimates are 
becoming clear, we may expect that with further research the range will 

narrow. . .,.,. 

If we can achieve a consensus of model results, convincing identi- 
fication of the projected C0 2 warming will increasingly depend on 
observational data, inspiring confidence and certainty sufficient to 
allow comparison with model results. This will require (1) that the 
necessary climatic data bases be accurate, comprehensive, and of suf- 
ficient length to allow application of appropriate statistical analyses 
and, in particular, identification of whether a change is or is not 



303 

occurring; (2) that data bases exist to evaluate the role of the factors 
that may have caused climate changes of comparable magnitude; (3) that 
the climatic effects of C0 2 and other factors be estimated with suf- 
ficient detail and confidence that the projected induced changes can be 
identified amid the fluctuations, perhaps similarly patterned/ caused 
by other known causal f actor s, unrecognized causes, and natural fluc- 
tuations (see MacCracken and Moses, 1982) . In the following subsections 
each of these points will be considered by example. 



5.2.2.1 Climatic Data Bases 

In the analysis of climatic change, and in particular in the search for 
evidence of C02-induced effects, one is faced with the limitations in 
the availability and accuracy of climatic data bases (see Table 5.2, 
for example) . 

As a result of these limitations, the primary indicator used in 
diagnostic studies of climate has been the change in surface air 
temperature during the last hundred years. (The problems illustrated 
and conclusions drawn in this section apply equally to analyses of 
variables other than temperature, such as sea ice extent and sea level 
rise.) This variable has been chosen because it is the only one for 



TABLE 5.2 Causes of Differences in Temperature Anomaly Data Sets 



(a) Differences Arising in Data Selection and Compilation 

Number of stations used in compiling average 

Methods for eliminating effects of unrepresentative stations 

(e.g., urban heat-island, station location changes) 
Relative distribution of stations over land and ocean 
Sources of data (ships, land sites, islands, for example) 
Treatment of stations with records starting in different years 
Differences or changes in observation times 
Unrepresentativeness of the sampling network 
Absolute temperature versus value of anomaly 
Methods of accounting for missing data 
Use of annual average versus monthly data 
Method of constructing daily average temperature. 

(b) Differences in Averaging Methods 

Time periods (range from annual to multidecadal) 

Spatial domains 

Climatic baselines and periods of record 

Time-averaging methods (running averages versus finite period 

averages) 
Spatial interpolation techniques (sum of representative sites, 

interpolation onto grid, weighting of stations, for example) 
Means of selecting normals and identifying trends 



304 



0.8 



0.4 



o 



-0.4 




I 



Jones etal. (1982) 
Vinnikovetal. (1980) 

. I . . I . 



1880 1895 1910 1925 1940 

YEAR 



1955 



1970 



1985 



FIGURE 5.3 Comparison of the reconstructions of annual surface-air- 
temperature anomalies for the northern hemisphere from Jones et al. 
(1982) and Vinnikov et al. (1980). Figure from Clark (1982) , but data 
for 1981 added to Jones et al. (P. D. Jones, Climatic Research Unit/ 
University of East Anglia, Norwich NR4 7TJ r England/ personal 
communication) . 



which a long record of measurements exists at many stations and because 
it is a convenient and straightforward, although not complete, measure 
of the climatic state. While .some investigators have attempted to 
develop data bases of global and polar temperature change, the most 
used indicator has been the change in northern hemisphere average 
surface air temperature. 

Comparison of the temperature records compiled by several inves- 
tigators shows that not only has temperature fluctuated but also that 
the temporal pattern of the temperature anomalies is not uniquely 
established (Figure 5.3). 

Estimates of surface-air-temperature anomalies using similar data 
sets and techniques agree well [e.g., Borzenkova et al. (1976), 
Vinnikov et al. (1980) and Jones et al. (1982)], although there remain 
differences in the details of the compilations that remain to be 
resolved (World Meteorological Organization, 1982a) . Hansen et al. 
(1981) compiled data on the global temperature pattern and found a very 
different pattern of temperature change in the southern and northern 
hemispheres (Figure 5.4). Jones et al. (1982) show that the changes 
have different patterns by season. 

The differences in estimates of actual climatic change arise 
primarily for two reasons: the choice of data selection/compilation 
methods and the choice of averaging methods. Table 5. 2 (a) lists some 
of the problems that arise in data selection and compilation; Table 
5.2(b) lists problems related to averaging techniques. Consider just a 



305 



Northern 
latitudes 
(90 N -23.6 N) 



Low latitudes (23.6 N-23.6S) 



Southern latitudes 
(23.6S-90S) 




YEAR 

FIGURE 5.4 Reconstruction of surface-air-temperature anomalies for 
various latitude bands by Hansen et al. (1981) , 



306 

few examples as illustrations of the problems. Mitchell (1961) used 
Willett's (1950) data and used an area weighting method rather than 
averaging of representative stations; this reduced the estimated 
wintertime climatic change over the previous 100 years by a factor of 
2. Yamamoto (1980) used both 30-year averaging (which might be appro- 
priate when considering the effect induced by the thermal lag of the 
ocean) and 5-year averaging; the time of maximum temperature and the 
start and pattern of the recent decline in temperature appear to be 
delayed about a decade when the longer averaging period is used. 
Paltridge and Woodruff (1981) used ship data on sea surface temperature 
(S8T) as an indicator of surface temperature in oceanic regions rather 
than the isolated island data used by most other investigators; they 
found the time of peak temperature delayed about 20 years and, quite 
surprisingly, a larger temperature change than shown by the land-based 
records. Their results, however, should be considered as only an 
initial examination of the SST data in that their averaging techniques 
for handling gaps in the record raise many questions. 

An additional complication in using available temperature records to 
estimate temperature changes induced by increasing C02 concentrations 
is that the records do not extend back to before the time that C0 2 
concentrations began to rise. In addition, we cannot be sure that the 
climate prior to the start of large anthropogenic C0 2 emissions was 
in equilibrium so that comparisons between early and late parts of the 
record may include biases due to trends, natural variations, or changes 
induced by non-C02 factors. 



5.2.2.2 Causal Factors 

The state of the climate system is determined by the interactions of a 
large number of processes and factors, some external to the system 
(e.g., solar radiance, aerosol and trace-gas concentration). A change 
in C0 2 concentration is only one of the factors that may induce a 
change in the climatic state. Moreover, climate models show that, even 
when all external factors are held constant, there will still be 
substantial climate fluctuation due only to interactions between 
various processes having different time scales. The fluctuations will 
be even greater when variations in external factors are allowed. 

While there are several potential sources of climatic change, our 
discussion concentrates for the moment on C0 2 concentration, volcanic 
aerosols, and solar variability, possible major influences that have 
been addressed by a number of investigators whose work deserves comment. 
In the future, other factors, such as changes in anthropogenic trace 
gases, may become influences comparable or greater in magnitude. 
Finally, we must recognize the possibility that some important factor 
may have been underestimated or remains unrecognized. 

To identify the climatic signal caused by increasing CO 2 concen- 
trations among the background of fluctuations requires either (a) that 
we treat the changes of all other factors as noise and wait until the 
C0 2 -induced climatic change is large enough to cause a statistically 
significant change (assuming implicitly that no additional competing 



307 

influences have come into play) or (b) that we account for the relative 
roles of the possible factors that may induce changes comparable in 
magnitude to increasing C02 concentration during the period of 
interest, thereby reducing the amount of unexplained variation and 
allowing easier identification of the C02 effect. In the record of 
the last 100 years/ the standard deviation of monthly global mean 
temperature ranges from 0.23 to 0.65C depending on month (Jones et 
al., 1982), while the change in annual global mean temperature is on 
the order of a half degree. Presumably the year-to-year variations 
were not due primarily to changes in C02 concentration. Since both 
observed variations and predicted changes are of similar magnitude, the 
latter approach of accounting for the role of as many causal factors as 
possible must be pursued if we are to have an early likelihood of 
identifying the projected C02 warming. 

To attribute climatic changes to causal factors requires adequate 
data bases for the changes in each causal factor, as well as for the 
climate state itself, over a period sufficiently long that changes 
attributable to the causal factors are comparable with the level of 
natural variability. Moreover, for carbon dioxide the record is not 
yet adequate. The mid-nineteenth-century baseline concentration is 
thought to have been between 250 and 290 ppm, and the time history of 
C(>2 concentration between 1850 and 1950, when changes in the bio- 
sphere may have played a more important role than fossil fuel emissions, 
is not yet well defined (cf . Chapter 2) . These uncertainties allow 
estimates of the change in CC>2 concentration from 1850 to 1980 to 
range from as little as 50 ppm to as much as 90 ppm, which in turn 
converts to a factor of about 2 in the estimated mean value of the 
warming attributable to changes in C02 concentration over this period 
if other factors remained constant. Combining the uncertainties caused 
by the range of possible change in C02 concentration, the range in 
model estimates of the temperature change for a CO 2 doubling, and the 
time constant of a thermal lag in the ocean, and using a logarithmic 
approximation to relate the C0 2 radiative effect to the temperature 
response, one finds that, assuming no net influence of other factors, 
the expected COp-induced temperature change since 1850 may range from 
a few tenths of a degree to more than one-and-a-half degrees; estimates 
in the lower part of this range appear more consistent with the 
climatic record (see Figure 5.5). 

The data bases used to estimate the role of volcanoes presumed to 
be an important factor in creating fluctuations over at least the last 
100 years are quite uncertain. Table 5.3 lists some of the factors 
contributing to the disagreements between the various data sets that 
have been used to account for changes in volcanically injected strato- 
spheric aerosols. The different records show quite different relative 
magnitudes and temporal patterns of volcanic influences. Lamb's (1970) 
dust veil index, a measure of atmospheric aerosol content, is perhaps 
the most frequently used index, but various investigators modify it, 
often in ad hoc ways, thereby perhaps affecting later calculations (see 
Figure 5.6). Gilliland (1982) uses acidity as measured in a Greenland 
ice core as the basis for variations in stratospheric aerosol, suggest- 
ing that the volcanic chronology from this record should record the 



308 



2.5 



2.0 



1.5 



1.0 



0.5 



AT, 



4.50 



3.75 



3.00 



2.25 



1.50 



I 




I 



I 



230 240 250 260 270 280 290 300 310 320 

t C0 2 ] 1850 

FIGURE 5.5 Relationship between CC>2 change/ temperature change, and 
climate sensitivity assuming no other forcings. The abscissa represents 
a range of values for mid-nineteenth-century C0 2 concentration. The 
ordinate represents the increase (AT?) in global mean equilibrium 
surface temperature between 1850 and the period 1961-1980. The response 
is calculated for a range of values of AT^, the change of global 
mean equilibrium temperature for a doubling of CC>2 concentration 
(assumed independent of initial CO 2 concentration) , and assumes that 
the temperature range is logarithmically related to the change in CC^ 
concentration (Augustsson and Ramanathan, 1977) . An ocean response 
time (mean thermal lag) of 15 years is used. The concentration of 
C02 was assumed in each case to increase linearly from the indicated 
value in 1850 to 310 ppm in 1950, and then linearly from 1950 to 340 
ppm in 1980. Note that if the temperature increase from 1850 to the 
interval 1961-1980 is taken to be 0.5C, then for consistency, AT^ 
may be as large as 4.5C only if mid-nineteenth-century C02 concen- 
trations were about 300 ppm, whereas AT^ may be as small as about 
1.5C if mid-nineteenth-century CO 2 concentrations were as low as 250 
ppm. For ocean response times shorter than 15 years, the isolines 
slide upward. Varying the time of the start of the increase in C0 2 
concentrations from 1850 to 1920 has little effect. 



309 

TABLE 5.3 Factors Contributing to Uncertainties in Creating a Volcanic 
Aerosol Data Base for Assessing the Climatic Effect of the Aerosol 

Reliance on variables not directly related to the radiative effect of 

volcanic aerosol (e.g., ejecta volume, volcanic explosivity r 

ice-core-derived precipitation acidity) 

Single or limited measurements extrapolated to global domain 
Lack of latitudinal resolution or pattern of aerosol distribution 
Assumptions about stratospheric lifetime of aerosols 
Lack of seasonal resolution of aerosol distribution 
Use of surface measurements to estimate changes in stratospheric 

aerosol (thereby introducing potential problems if trends exist in 

tropospheric aerosols) 

Averaging period of volcanic aerosol loading 

Definitions of major volcanoes and of the number of volcanoes considered 
Assumptions about size distribution of volcanic particles and of 

chemical composition of gaseous emissions, leading to ad hoc 

adjustments 
Assumptions regarding lifetime and distribution of aerosol due to 

season, latitude, and height of injection of volcanic dust and gases 
Circularity caused by estimation of dust loading from observations of 

subsequent temperature change 



high northern latitude volcanoes often cited as missing by other 
investigators but strangely not evident in the core record. The recent 
Smithsonian compilation of volcanoes has also greatly expanded the 
number of volcanoes considered (Simkin et al., 1981). New listings, 
however, do not always reinforce confidence. For example, the recent 
compilation of an explosivity index by Newhall and Self (1982) 
attributed greater importance to the eruption of Mt. St. Helens than to 
Agung, whereas a sulfur-based stratospheric aerosol index would place 
Agung as being of much greater importance than Mt. St. Helens. 

In addition to estimates of dust amount, a few indices also rely on 
actual radiative measurements (Figure 5.7). The general pattern of the 
two types of data show broad qualitative similarity, but details of 
timing and magnitude are quite different. 

Even worse complications exist in generating data bases to evaluate 
the effect of changes in solar radiation, the third major factor often 
considered in these analyses. For this factor, the main problem is 
that there are several indicators that have been used as surrogates of 
the solar radiative flux reaching the Earth, but there is virtually no 
physical basis for deciding between these indicators, or even if any is 
relevant (i.e., is the solar constant constant?). 

Quite clearly, without adequate historical data bases on the causal 
factors, identification of the CO 2 part of the climatic signal may 
not be possible. Therefore, examination and improvement of the data 
bases on potential causal factors must be an essential part of the 
early detection effort. 



IT 



310 



( V) stratospheric loading by volcanic activity 



Adjusted to northern hemisphere 



120-yr average V 
4.2 x 10 6 tons 




II 

QC I- 



QO 

III \J 



20 
10 



Krakatoa 



Bandai San, 
Bitter Island 

Awu 




1880 



*Aw 
I A 



1900 



Mont Pelee, Soufriere 
Santa Maria 

Shtyubelya Sopka 



I 
1920 

YEAR 




Fernandina ~i 

Awu 
Surtsey 
Agung 

Bezymyannaya 
Mt. Spurr 
Hekla 




1940 



1960 



LU 

I 

D 



50 



X 

LU 
Q 



100 



150 



200 



250 




I 



1890 



1910 



1930 
YEAR 



1950 



1970 



FIGURE 5.6 Estimates of stratospheric aerosol loading by (top) 
Mitchell (1970) and (middle) Oliver (1976) based on use of volcanic 
index of Lamb (1970) and by (bottom) Bryson and Dittberner (1976) using 
volcano index of Hirschboeck (1980) . The model of Oliver (1976) uses a 
residence time approach to calculate aerosol loading after injection. 
The curve of Bryson and Dittberner (1976) represents a 10-year running 
mean in arbitrary units. 



311 



LU 

E 
Q 
LU 



DC 
UJ 



104 



102 



100 



02 
:: o 




1890 



1900 



1910 



1920 
YEAR 



1930 



1940 



I 
1950 1960 



0.12 



t 0.10 

LU 

o 



a. 
O 



0.08 



0.06 



0.04 



0.021 
1880 




1900 



1920 



1940 



1960 



1980 



YEAR 



FIGURE 5.7 Estimates of stratospheric aerosol loading based on surface 
measurements of downward direct solar radiation (top) from Budyko 
(1969) using data from cloudless days of each month with 10-year 
smoothing, and (bottom) from Bryson and Goodman (1980) using data from 
42 stations between 20 and 65 N. The apparent inverse relationship 
occurs because direct radiation measures the clarity of the stratosphere 
whereas aerosol optical depth measures the lack of clarity. 

5.2.2.3 Relating Causal Factors and Climatic Effects 

Even given precise records of how the climate has varied and the 
history of the important causal factors, a number of problems arise in 
attributing appropriate components of the climatic fluctuations of the 
last 100 years to the various causal factors, the remainder of the 
fluctuations being assumed to be natural (or, more properly, natural 



i 



312 

TABLE 5.4 Limitations in Determining the Relationship between Changes 
in Causal Factors and Changes in Climate 



Imperfect or incomplete modeling of relevant climatic processes (e.g., 

with respect to the oceans and cryosphere) 
Imperfect model verification with respect to factors of interest (e.g. , 

lack of sufficient test cases to verify treatment and time constants 

of long-term processes and of model performance as a whole) 
Limited areal extent of analysis (e.g., less than three-dimensional, 

less than global) 
Use of equilibrium rather than the transient perturbations in 

developing relationship between climate and causal factors 
Internal variability of climate models 
Limited length of model simulations 
Different times at which analysis starts (for example/ the decade of 

the 1880s was strongly volcanically affected; a stable baseline may 

not be available) 
Different temporal patterns of assumed climatic response functions 



plus as yet unexplained). For the attribution to be convincing, the 
change in the causal factor must be physically related to the climatic 
change in a quantitative way, which requires both theoretical under- 
standing of how the causal factor affects the climate and the ability 
to calculate the effect numerically. Since there is more than one 
causal factor, we must also be able to calculate the effects of inter- 
actions when more than one causal factor is acting. Table 5.4 lists 
some of the problems that arise in attempting to calculate the 
relationship. 

As an example of the type of problem that exists, consider the 
results of different approaches for treating the volcano-climate 
coupling used by different authors. Although Vinnikov and Groisman 

(1982) and Bryson (1980) both indicate that they use the same 
surface-air-temperature record and similar actinometric measures of 
stratospheric transmissivity, their different approaches to relating 
the causal factor to the induced climatic change lead them to very 
different conclusions about whether there has or has not been a 
temperature increase attributable to increasing C0 2 concentrations. 
Rather than specifying a response function, Oliver (1976) and Mitchell 

(1983) chose generalized volcanic response functions that were then 
optimized in order to fit the temperature data. They also come to 
differing conclusions about the role of CO 2 in the temperature record. 

Rather than employing such empirical and statistical approaches, 
Hansen et al. (1981) used a one-dimensional radiative-convective model 
with a thermodynamically interactive ocean to simulate the climatic 
response to changes in the volcanic aerosol loading (and other factors) . 
They then derived a quantitative relationship between C02 concentration 
and temperature to use in their analyses of the causes of climatic 
fluctuations. Such use of physically based relationships is an 



313 

important requirement for studies of this type. Comparison of their 
one-dimensional calculations with the different time histories of the 
northern and southern hemisphere temperatures , however, makes clear 
that there is a need to analyze the observed fluctuations with models 
having, at least, greater spatial detail. 

Such complications as arise in relating volcanic eruptions to the 
temperature response also arise in attempting to relate other causal 
factors to temperature change. In some cases, these complications can 
be resolved by just expanding the models to include omitted, but 
important, processes and domains or to treat the transient as opposed 
to the equilibrium response; in other cases, the complications can only 
be resolved by improving existing data bases (e.g., extending the 
temperature record back in time) or gathering new data (e.g., changes 
in Antarctic ice volume, if our climatic variable is sea level) . 

As illustrations of the difficulty in untangling the recent climatic 
record, the next section considers several recent attempts to acquire 
the necessary climatic and causal factor data bases and then to relate 
these data bases to climatic changes. 



5.2.3 Attempts to Identify COp-Induced Climate Change 

Mankind has always sought to relate climate fluctuations to causal 
factors. Certainly, the seasonal variations of solar position were 
widely recognized by early man as being related to the seasonal vari- 
ations in temperature, even if the reasons for the change in solar 
position were not understood. Geologists once feared that the Little 
Ice Age was caused by the cooling of the sun as its fuel sources ran 
down, until they found evidence for very cold periods thousands and 
then millions of years earlier. Since discovery of these ice ages, 
there have been many suggestions about the causes of climate change. 
Current views of many scientists, for example, are that the sun's energy 
output has actually been increasing over geological time, that vari- 
ations in the Earth's orbital parameters were major factors in the 
Pleistocene glacial cycles, and that the cold decades that characterized 
the Little Ice Age resulted from small fluctuations in solar output. 

Although changes in C02 concentration were recognized as a possible 
factor in long-term climate change during the last century, the work of 
Callendar described earlier has served as the basis for numerous studies 
on that subject during the last 20 years (see Table 5.5). These studies 
have used different data sets, different analysis techniques, con- 
sidered different causal factors and, perhaps not surprisingly, reached 
different conclusions. Because of these many differences, comparing 
the results is not straightforward. 

As one measure of the differences in findings, Table 5.6 presents the 
results of a representative set of investigators in terms of a ratio 
measuring the relative importance of each causal factor in causing a 
climatic change to the maximum variation of the climatic variable in 
the record that was used. In making these comparisons, it should be 
noted that in many of the studies there has been some effort to achieve 
an optimum fit to observations by choice of data base or by adjusting 



314 




TABLE 
Tempe 



315 



0> 



M 

e.S 



T3 

<d 



M 

^ 

O ft) 

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318 

the strength of the relationship between variables, i.e., by model 
"tuning" or "curve fitting." 

The factors considered by the studies included in Table 5.6 include 
C02, volcanic (stratospheric) and tropospheric aerosol loading, and 
various suggested measures of the variation in solar radiation reaching 
the top of the atmosphere. The results indicate that, depending on 
investigator, each of these variables can separately account for any- 
where from nearly all to none of the observed climatic variations of 
the last 100 years. In addition, when the coupled effects of more than 
one factor are considered, there remains a significant range in the 
ratios. 

Closer examination of a few of these studies shows more clearly the 
uncertainties and discrepancies that are involved, although identifying 
the exact causes of these disagreements is beyond the scope of this 
report. 

In comparing the various studies, consideration should be given to 
how well their choice of causal factors is able to explain the major 
features of the climate of the last hundred years as indicated by almost 
all available data sets, including the warmth of the northern hemisphere 
from 1920 to 1950 and the relatively steady warming of the southern 
hemisphere . 



5.2.3.1 Carbon Dioxide as a Causal Factor 

If we consider every influence other than CO2 that affects the climate 
to be part of the natural climatic variability, we can attempt to 
identify the C02 signal amid the natural fluctuations, or "noise," of 
the climate record. The level of noise that can, to first order, be 
estimated by examination of climatic records before CC>2 is thought to 
have had a substantial climatic effect. Past C0 2 concentrations have 

been frequently estimated by extrapolating back from current concentra- 
tions assuming that inputs from fossil fuel combustion have been the 
dominant source and that the fraction of each year's emission remaining 
airborne has been constant. If fossil fuels have been the primary 
factor perturbing atmospheric CO2 concentrations, then the period 
prior to perhaps 1930 can be used as a baseline. In this case, a 
reasonably long record is available, and the variability of annual 
average temperatures may be estimated with some reliability to be a few 
tenths of a degree. If, however, the biosphere has been an important 
net source of atmospheric C(>2 over the last 150 years, or parts 
thereof, then the C0 2 -induced trend in temperature may not now allow 
selection of an adequate baseline from which to estimate natural 
climatic variability. 

Hansen et al. (1981) calculated the effect of a C0 2 increase on 
the global climate of the last 100 years, using a baseline C0 2 con- 
centration of 293 ppm (which might be too high in 1880 if biospheric 
sources have been important) and assuming that a doubling of C02 
concentrations would lead to an equilibrium warming of 2.8C. As shown 
in Figure 5.8, their model-computed response to the CO 2 increase 
alone did not compare well with the global record. Interestingly, 



319 



0.4 



0.2 






-0.2 



-0.4 




Model (C0 2 ) 
._ M _ Observations 



1880 



1900 



1920 



1940 



1960 



1980 



YEAR 



FIGURE 5.8 Comparison of observed global mean temperature anomalies of 
the last 100 years with anomalies predicted by a one-dimensional 
climate model assuming only that CC>2 concentrations are varying 
(Hansen r 1980) . 



however, although their result is in poor agreement with the northern 
hemisphere temperature record, it is in rather good agreement with the 
low-latitude and southern hemisphere temperature records (refer to 
Figure 5.4). These differences point to the need for conducting future 
analyses in more than one dimension; it may be, for example, that 
because of fewer volcanoes in the southern hemisphere and because of 
the thermal inertia of southern hemisphere oceans, the C(>2 warming 
may more easily be found in the southern rather than northern 
hemisphere. 

Madden and Ramanathan (1980) also attempted to identify the CO2 
climatic effect, assuming that all other factors contributed to the 
climatic noise. They looked in the 50-60 N latitude band where the 
spatial coverage of the records is quite good and where equilibrium 
climate models predict that the temperature changes should be largest* 
Their results were negative (i.e., they found no statistically sig- 
nificant signal emerging from the noise), indicating, they suggested, 
that either the attempts to estimate present temperature changes from 
equilibrium climate models are inadequate or that the models are over- 
estimating the CC>2 warming. Wigley and Jones (1981) also did not 
find evidence of a C0 2 signal when examining records of the northern 



320 

hemisphere surface temperature; they also did not attempt to reduce the 
noise by considering the possible climatic effects of other factors. 

The failure of these and earlier efforts to identify unequivocally a 
C02 signal in the noisy global temperature record suggests that 
attempts should be made to take into account other causal factors in 
order to reduce the residual variance , and thus to make a hypothesized 
C02 signal stand out more clearly. 



5.2.3.2 Volcanic Aerosol as a Causal Factor 

Several authors have considered how to relate changes in volcanic 
injections of stratospheric aerosol to climatic fluctuations of the 
last 100 years. Model results of Oliver (1976), Robock (1978) , and 
Bryson (1980) all suggest good agreement between observed temperature 
anomalies and volcanic forcing, finding no requirement for consideration 
of C0 2 or solar effects. The model and data used by Hansen et al. 
(1981), however, found less satisfactory agreement (see Figure 5.9). 
The primary difficulty in achieving good agreement with just volcanic 
forcing is in explaining the temperature changes in the northern 
hemisphere, in particular the warming from the 1920s to 1930s and 
cooling from the 1940s to 1970s. The strength of these features varies 
depending on the temperature record used and the treatment of temper- 
atures over land and over ocean. Hansen et al. (1981) indicate that 
the anomalous warm period in the northern hemisphere was about 0.4C 
above a trend line through the rest of the record, whereas in Jones et 
al. (1982) and Budyko (1969) the anomaly is slightly less. Although 
most authors use the northern hemisphere temperature record as a basis 
for evaluation of the relationship between causal factors and climate 
changes, Hansen et al. (1981) used a global record that somewhat reduces 
the intensity of the 1920-1950 warming period that must be explained 
because of averaging in the rather steady warming trend in the southern 
hemisphere. 

Bryson (1980) believes the problem that most investigators have in 
explaining the cooling that occurred prior to the Agung eruption of 
1963 arises for two reasons. The first is the incompleteness of Lamb's 
(1970) volcanic record? in response to this criticism several volcanic 
eruptions in the 1950s have been added to most volcanic chronologies. 
The second reason is the inadequacy of estimates of the radiative effect 
of volcanic injections by the traditional methods of estimating the 
amount of dust injected. The recent El Chichon volcano and analyses of 
the effect of the Agung eruption have emphasized that the injection of 
sulfur-bearing gases rather than total dust is likely the more appro- 
priate measure of the ultimate radiative effects. Although probably 
also affected by tropospheric aerosols, Bryson and Dittberner (1976) 
and Budyko (1969) urge use of actinometric data as a better measure of 
stratospheric turbidity. 

It is also difficult to reconcile the relative effects on average 
temperature in the northern and southern hemisphere based on volcanic 
records, especially during the 1900-1910 period when (according to the 
temperature reconstruction of Hansen et al., 1981) the supposedly less 



321 



0.6 

0.4 

o 0.2 






-0.2 
-0.4 




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(T - T Q ) 1883 = 0.4, R = 1.0. C = 6.5, a - 0.162 



1880 



Bezymyannaya and Agung at 10 




1960 



-0.2 - 



o 




1880 



1900 



1920 



1940 



1960 



1980 



YEAR 



FIGURE 5.9 Comparison of observed surface-air-temperature anomalies of 
the last 100 years with anomalies predicted by models that included 
only changes in stratospheric aerosol loading as calculated by (top) 
Oliver (1976) and (bottom) Hansen (1980) . Oliver compares his results 
with the northern hemisphere temperature record and Hansen to the 
global temperature record. 



responsive southern hemisphere cooled more and prior to the northern 
hemisphere , even though listings of major volcanic activity (e*g./ 
Mitchell, 1970) indicate that all important eruptions were in the 
northern hemisphere. 

While the case for coupling between volcanic activity and climate is 
suggestive, the data bases and analysis methods still need much work in 
order to be able to account accurately for the climatic effects of 
volcanic injections with confidence. Analysis of the climatic fluctua- 
tions subsequent to the El Chichon eruption will be an important aspect 
of this effort. 



322 
5 2.3. 3 Solar Variations as a Causal Factor 

The identification of cycles in various climatic data bases having the 
same periodicity as such indices of solar activity as sunspots has led 
to extensive efforts to attribute various climatic fluctuations to solar 
variations (Eddy, 1977; Geophysics Study Committee f 1982). The absence 
until very recently of both physical measurements to corroborate the 
value of the surrogate solar indices and of detailed explanations of 
the cause-effect relationships are important caveats to remember. 

Studies of sunspot-climate relationships have a long history. Of 
particular interest in recent years has been the apparent absence of 
sunspots (the Maunder minimum) noticed during the coldest periods in 
Europe in the 1600s. In detailed analyses, however, Mass and Schneider 
(1977) and Robock (1978) found little statistical significance in the 
in-phase relationship between Wolf sunspot number and temperature. 
This has led to consideration of other surrogate indices of solar 
activity. 

Robock (1978) considered the suggestions of Eddy (1976) that the 
solar output is a function of the alternate 80- and 100-year cycles 
(the Gleissberg cycle) that seem to modulate the amplitude of the 
sunspot cycle. Robock did not find any statistically significant 
agreement using this index. Although Mitchell (1961) had found some 
agreement with the observed temperature record up to 1940 if he 
considered temperature changes due to both C0 2 and time-averaged 
sunspot number, these factors could not explain the later cooling that 
was observed. Eddy et al. (1982) suggest that the fractional areal 
coverage of the solar disk by sunspots, rather than actual sunspot 
number, is the appropriate index of solar variability. 

Hoyt (1979a,b) finds apparently good agreement between the northern 
hemisphere temperature record and the umbral/penumbral ratio, a possible 
measure of the convective energy transport in the sun's photosphere, 
and therefore of its time-varying radiant flux. The range of his 
results was calibrated to agree with the temperature record; note that 
he found that umbral/penumbral ratio and C0 2 forcing, and not vol- 
canic forcing, were sufficient to reproduce the variations in the 
observed temperature record. It would seem that, if valid, the solar 
effects should be similar in temperature records of both hemispheres 
except for the influence of differing amounts of ocean; the differences 
in the temperature records that exist would, it would seem, cast some 
doubt on this explanation. 

Hansen et al. (1981) find that including the umbral/penumbral ratio 
improves their fit when also considering C0 2 effects (compare Figure 
5.10, top, with Figure 5.8). 

Gilliland (1981) uses the variation in solar radius as an alternative 
measure of solar irradiance. This cycle is about 76 years in length 
and is negatively correlated with the Gleissberg cycle. There are 
indications that the Greenland ice core exhibits cycles with a length 
about the same as the solar radius and Gleissberg sunspot cycles. 
Broecker (1975) suggested that the cooling associated with the 80 year 
Greenland ice core cycle was counteracting the warming expected from 
increasing CO 2 , leading to only slight change in present temperatures. 



323 



0.4 



0.2 



O 



-0.2 



-0.4 




Model (sun + COJ 
Observations 



0.5 



O 

o 



-0.5 




I 



Model 

Observations 

i 



1880 1900 1920 1940 

YEAR 



1960 



1980 



FIGURE 5.10 Comparison of the observed change in surface air 
temperature and model predictions of the change in temperature when 
considering the increase of CO 2 concentrations and changing solar 
radiance. (Top) Hansen (1980) uses umbral/penumbral ratio as an 
indicator of solar radiance and compares with global temperature 
change. (Bottom) Gilliland (1982) uses changes in solar radius as an 
indicator of solar radiance and compares with northern hemisphere 
temperature change. 



1 



324 

This suggestion of a balancing of opposing effects of different causal 
factors is also evident in Gilliland's (1982) analysis, in which there 
could be no accommodation of C02 warming were it not for the postu- 
lation of solar-induced cooling in the last few decades (Figure 5.10, 
bottom) . 

Given the lack of direct observational support for the suggested 
relationships between the surrogate solar variables and solar radia- 
tion, it seems premature to make quantitative estimates of the C02 
effects on the basis of hypothesized reduction in solar radiance. At 
the same time, one must recognize that solar variations may mask the 
C0 2 effect. It is also interesting that there is a similarity in 
character between the various surrogate indicators of solar activity 
(solar radius variations, Gleissberg cycle, and smoothed umbral/ 
penumbral ratio) and the actinometric and smoothed dust veil indices 
that are assumed to represent volcanic activity (e.g., see Siquig and 
Hoyt, 1980) . This may, in part, explain why different investigators 
using different data bases have reached similar conclusions. 



5.2.3.4 Combinations of Causal Factors 

Because single factors have difficulty in explaining the records of 
climatic fluctuations, a combination of factors has often been used. 
Some authors (Hansen et al., 1981; Vinnikov and Groisman, 1981, 1982; 
Mitchell, 1983; Gilliland, 1982) find that volcano and C0 2 effects 
lead to reasonable fits to the data (Figure 5.11), although it should 
be remembered that some other authors do not require C0 2 to achieve 
an equally good fit, depending on how volcanic effects are included. 
Vinnikov and Groisman (1981) also indicate, for example, that use of 
the stratospheric aerosol series of Lamb (1970) and Mitchell (1970) 
instead of that of Budyko (1969) leads to a qualitatively different 
conclusion that there is no significant influence of C0 2 on climate 
change over the last 100 years. Thus, even among those considering 
just volcanic and CO 2 effects, there is disagreement on the role of 
each factor. 

Hansen et al. (1981) and Gilliland (1982) have achieved what appear 
to be excellent fits to the temperature record by considering C0 2 , 
volcanic injections, and solar variations (Figure 5.12). While the 
similar conclusions of these two studies may appear to be re-enforcing 
(see CO 2 /Climate Review Panel, 1982), it is disturbing that their 
data bases and relationships are quite dissimilar and in some instances 
contradictory. 

(a) The maximum variation of Hansen et al.'s global temperature 
record is about 0.5C, whereas Gilliland's northern hemisphere 
temperature record has a range of about 0.9C. 

(b) Hansen et al.'s volcanic record (from Lamb) has major peaks for 
Krakatoa (1883) , Souf riere/Santa Maria (1902-1904) , and Agung (1963) . 
Gilliland's volcanic record appears to have major peaks for Askja 
(1875), unidentified (1885-1886), Katmai (1912), and Surtsey 
(1963-1965), most of which are high-latitude or local volcanoes. 



325 



0.4 



0.2 



o 



-0.2 



-0.4 




Observations 
Model 



1880 



1900 



1920 1940 

YEAR 



1960 



1980 



0.5 



o 



-0.5 



i \ 1 r 

Observed (Borsenkovaetal.) 
- Fitted (Volcano + CO 2 ) 



T 




1880 1890 1900 1910 1920 



I I I 
1930 1940 1950 
YEAR 



L_ I .__ 
1960 1970 1980 



FIGURE 5.11 Comparison of the observed change in surface air 
temperature and model predictions of the change in temperature when 
considering the increase of C02 concentrations and changing 
stratospheric aerosol loading. (Top) Vinnikov and Groisman (1982) 
compare their results with data for the entire northern hemisphere from 
Vinnikov and Groisman (1981) ; (bottom) Mitchell (1983) compares with 
data north of 17.5 N. 



(c) Hansen et al. show a C0 2 effect beginning in the 1880s, 
whereas Gilliland's C0 2 effect does not start until 1925. (The 
initial concentration of C0 2 assumed by each of these investigators 
is also 5-10% above recent estimates of C0 2 concentrations in the 
last century.) 

(d) Hansen et al. use Hoyt's quite variable umbral/penumbral record 
as a measure of changes in solar radiance. Gllllland depends strongly 
on a smooth solar radius cycle of about 76 years and also includes solar 
variations having cycles of 12.4 and 22 years; phase and amplitude of 
the solar cycles were arbitrarily determined to provide the best fit 
with the temperature record. 



326 



0.4 



0.2 



_ 

o 

o 



-0.2 



-0.4 



-0.6 



0.5 




Model: sun + CO 2 + volcanoes 



O 
o 



-0.5 



Model 
Observations 



-L. 



1880 1900 1920 1940 

YEAR 



1960 




1980 



FIGURE 5.12 Comparison of the observed change in surface air 
temperature and model predictions of the change in temperature when 
considering the increase of C(>2 concentrations and changes in solar 
irradiance and stratospheric aerosol loading. (Top) Hansen (1980); 
(bottom) Gilliland (1982) . 



327 

These and other similar studies (Hansen et al., 1981; Vinnikov and 
Groisman, 1982; Gilliland, 1982; for example) acknowledge uncertainties 
in their presentation of data and their formulation of conclusions. 
They have been very helpful in raising questions, suggesting relation- 
ships t and identifying gaps in our data bases and observational 
approach, and we cannot preclude the chance that at least one may be 
correctly relating causal factors and temperature changes. However/ 
contrasting causal components of the climate change and differences in 
data bases make it difficult to accept the results as reinforcement of 
the general hypothesis of the C0 2 -induced climate shift. 



5.2.4 Steps for Bui Id in - Confidence 

Are results to date sufficient basis for declaring that a climate change 
due to increased C(>2 has already occurred? Given the potential impor- 
tance of such a finding, we must require a high standard of agreement 
in attributing climatic effects to causal factors. 

Methodologies for determining the statistical significance of sug- 
gested climatic changes have recently been discussed by Madden and 
Ramanathan (1980) , Wigley and Jones (1981) , Klein (1982) , Epstein 
(1982), Hayashi (1982), Murphy and Katz (1982), Katz (1980, 1981), and 
an international group (World Meteorological Organization, 1982a) . 
Epstein, for example, decomposes the time series of observations of a 
climatic variable into three components: a "natural" climatic mean, a 
possible climatic change induced by some extrinsic factor such as 
C0 2 , and a random variability. The natural mean may be taken to be 
constant, or it may include the estimated influences of some forcing 
factors, e.g., solar or volcanic activity. Various hypotheses 
regarding the climatic change may now be stated and tested. 

The conclusions to date have depended crucially on the assumptions 
made regarding the underlying climatic trend, the increase of C0 2 
between the raid-nineteenth century and recent times, and the expected 
C0 2 influence. Once the hypothetical C0 2 signal is prescribed on 
the basis of estimated C0 2 concentrations and model simulations, the 
remaining variance of the climatic record is partitioned between the 
fluctuations explained by other external factors and the unexplained 
fluctuations deemed to be "noise." Reduction in the unexplained 
variance relative to the variance explained by C0 2 is taken as 
supporting the hypothesis of C0 2 influence. It is difficult to draw 
unambiguous conclusions from studies of this type for the following 
reasons: < 

1. The natural variability of global mean temperature is imper- 
fectly known because of the relatively short period of instrumental 
record. Moreover, the spectrum of observed climate variability exhibits 
considerable power at low frequencies. In other words, natural climate 
variations have been observed on time scales commensurate with the time 
scale of CO 2 increase. At least in part, these may be attributable 
to external factors whose influence might, in principle, be removed. 
However, an unknown and possibly large amount of natural variability or 



328 

noise might well remain at low frequencies, making it difficult indeed 
to distinguish over short periods of record between slow trends induced 
by increasing CC>2 and equally slow trends that reflect the natural 
low-frequency variability of the climate system. For this reason, the 
hypothesis that the observed variability is entirely due to natural 
causes cannot be unequivocally rejected on the basis of the studies 
conducted so far. 

2. The global mean temperature record has been reconstructed from a 
relatively short and geographically limited set of observations, pri- 
marily over land areas of the northern hemisphere. The period of record 
spans a period of great change and expansion in the human societies that 
make and record the observations. While investigators have conscien- 
tiously worked to remove or correct human influences such as urbaniza- 
tion on, for example, temperature records, it is difficult to assess 
their degree of success. The accuracy and representativeness of the 
data are thus open to question, although the broad trends are believed 
to be reliable. 

3. Solar radiance variations have been estimated from surrogate 
observations, e.g., sunspots, umbra/penumbra ratios, and solar radius 
measurements, whose relationship to solar ir radiance is by no means 
clear. Moreover, the relationship between solar variability and 
climate has been determined only empirically from a limited data base. 

4. Atmospheric turbidity has been estimated from data on volcanic 
activity (e.g., individual eruptions or the "Dust Veil Index," or 
acidity measurements in ice cores) or from actinometric observations. 
None can be considered very reliable. As with solar variations, 
turbidity influences have been only empirically fitted to a limited 
record. 

5. The history of atmospheric CO2 concentrations before 1958 is 
poorly known, with estimates of mid-nineteenth-century concentrations 
ranging from 250 to 290 ppm. The expected temperature change is thus 
correspondingly uncertain. 

6. Other neglected factors, e.g., surface albedo, aerosols, and in 
recent years anthropogenic trace gases, may have influenced the 
temperature record. 

7. The choice of the appropriate signal of CC>2-induced climate 
change is by no means clear; many results from models of different 
degrees of physical completeness are available (Schlesinger, 1982, 
1983). Moreover, model results are available only for equilibrium 
conditions, while the real world is presumably exhibiting a transient 
response to increasing CO 2 . The transient response will be greatly 
complicated by the thermal inertia of the ocean and the distribution of 
ocean and land on the Earth's surface. Thus, real-world transient 
responses might be quite different in their regional details from those 
that might be inferred from equilibrium models (see, Thompson and 
Schneider, 1979; Schneider and Thompson, 1981; and Bryan et al., 1982). 

8. Such studies are prone to a familiar pitfall of statistical 
inference, namely, the testing of multiple hypotheses. As noted by 
Epstein (1982) , "If enough different hypotheses are examined, then, by 
chance, it is likely that statistics supporting one of them will be 
found." 



329 

9. The "noise" accounted for by both external factors omitted from 
the analysis and unexplained sources is not independent from one time 
of observation to another f nor can it be reasonably modeled as a first- 
order Markov or autoregressive process. (If it could be so modeled, 
then our best prediction of next year's climate would only involve the 
present single year's data corrected for changes in the included 
external factors; in fact, however/ averages over many preceding years 
prove to be the best climatological estimates.) Thus, statistical 
techniques that are more careful and sophisticated than those for white 
noise or first-order autor egress ion will be required to deal with the 
increase of fluctuation energy at lower frequencies. 

There is a methodological point that should be made here with regard 
to claims to have detected a C(>2-induced warming based on a "success- 
ful" model fit to the climatological record (e.g., Hansen et al., 1981) 
as opposed to the simpler approach of identifying a warming signal 
rising above the "noise" of intrinsic climate variability (e.g.. Madden 
and Ramanathan, 1980) . Tests of statistical significance are required 
in both approaches. However, these tests have usually been made only 
in the latter, partly because significance tests for model fits are not 
generally to be found in textbooks and, indeed, must be developed 
separately for each model type. Until such tests have been devised and 
carefully applied, the scientific community will remain skeptical of 
claims to have detected with statistical confidence a CC^-induced 
signal in a single parameter such as temperature. 

Notwithstanding methodological difficulties, earlier detection may 
be sought in two ways. First, we may attempt to improve the objectiv- 
ity and physical basis of the estimates of external influences on the 
past climatic record, thus constraining the range of plausible hypoth- 
eses that could be tested. Needs are the following: 

1. Better determination of the natural variability of temperature, 
particularly at low frequencies by extending the period of record back 
in time through use of proxy records and by distinguishing between 
ocean and land records. 

2. Improvement in the accuracy and representativeness of the tem- 
perature record through incorporation of marine data and continued 
attention to influences such as urbanization. 

3. Better data bases on possible changes in solar output and 
atmospheric aerosol loading. 

4. Better reconstruction of the changes in C(>2 concentration over 
the last hundred years. 

5. Objective, physically based, and observationally validated 
relationships between solar variability, volcanic aerosols, other 
possible factors and climate. 

A second recently suggested approach is to attempt to isolate a 
pattern of changes specifically attributable to increasing C02 
concentrations (MacCracken and Moses, 1982) . This approach is 
discussed in the following sections of this chapter. 



330 

5.3 A STRATEGY FOR MONITORING CO 2 - INDUCED CLIMATE CHANGE 
5*3.1 The "Fingerprinting" Concept 

Proposals for monitoring programs to detect the effects of increasing 
C0 2 date back to the SCEP (1970) and SMIC (1972) reports. Recently, 
participants at a Department of Energy sponsored workshop on First 
Detection of Carbon Dioxide Effects (Moses and MacCracken/ 1982) 
proposed a three-part framework for detection of C0 2 effects 
involving: 

1. Identification of changes, 

2. Identification of possible causative factors/ 

3. Isolation of the parts of the changes attributable to increasing 
C0 2 . 

With respect to the last of these, a suggestion was made ". . .to 
develop a unique CO 2 -specific 'fingerprint 1 for the C0 2 response 
involving a set of several parameters, distinctive from responses that 
would be caused by all other known influences, and to search for this 
correlated pattern of changes, not just for a change in one isolated 
parameter." (MacCracken and Moses, 1982.) 

The concept of fingerprinting is based on the notion that a 
composite index based on multivariate statistical analysis of several 
parameters in space and time might enable us to attribute more 
positively climatic changes to increased CO 2 . Indeed, changes in 
some climatic elements might help to distinguish the effects of C0 2 
(or CO 2 in combination with other radiatively active gases) from 
those due to variations of some of the other factors or external 
conditions that could influence climate. For example, one might 
anticipate that the pattern of tropospheric temperature changes caused 
by turbidity variations due to volcanic aerosols would differ from 
those caused by the more globally uniform variations of C0 2 . 

While the notion of an index that would unequivocally reveal the 
influence of CO 2 on climate is indeed enticing, its practical 
application does not appear immediately feasible. The relationships 
between atmospheric CO 2 and climate variables must be deduced from 
model simulations. Simulations of the equilibrium response to highly 
elevated levels of C0 2 show considerable scatter in results (see 
Schlesinger, 1982, 1983), and simulations of the transient response to 
slowly changing CO 2 have not yet been accomplished. Even if a 
plausible index could be deduced from models, its statistical char- 
acteristics in the appropriate frequency range would need to be 
assessed through study of past data and through model simulations. 
Appropriate multivariate statistical tests would then have to be 
designed and applied. These difficulties must be overcome before a 
scientifically rigorous monitoring and detection strategy based on this 
approach can be devised and applied to provide clear guidance to 
policymakers. 



331 

Nevertheless/ the concept of a "C(>2 fingerprint" provides useful 
guidance in the design of a geophysical monitoring program that will 
both provide data for research and help us to follow the course of 
climate. We thus suggest that monitoring programs designed to shed 
light on the effects of increasing CO 2 should focus on the climatic 
variables whose patterns of change in space and time are indicated by 
models to respond most strongly to 002 increases, together with those 
external factors that may also influence climate particularly those 
external factors with greatest influence on the selected climatic vari- 
ables. The remainder of this chapter presents some specific suggestions 
based on this approach. 



5.3,2 Considerations in Climate Monitoring 

5.3.2.1 Statistical Variability and Expectations of Change 

All climatic parameters are highly variable in space and time. Their 
variability or "noise" arises both from systematic physical processes 
whose effect could in principle be calculated and from the random 
fluctuations of a turbulent atmosphere. The characteristics of the 
variability, including its preferential occurrence in one frequency 
range or another, can in some cases be determined from observed data, 
especially for climatic parameters at the Earth's surface such as air 
temperature and precipitation. Direct estimates of variability may in 
other cases have to be supplemented from the data simulated by compre- 
hensive climate models (Manabe and Hahn, 1981) . 

Estimation of the climatic change expected to result from increased 
C0 2 depends on climate models that address the processes and scales 
of interest. Estimation of the changes to be expected from increased 
C02 should therefore include their seasonal and geographical charac- 
teristics, their longer-term and arealy averaged properties, and their 
evolution in time as C0 2 concentrations increase. Some preliminary 
information of this sort is already available; extended simulations are 
required to provide more complete information. 

A schedule of expected climatic changes is especially necessary. 
Most experiments that have been made to date have estimated the response 
of climate from the difference between two simulated equilibrium 
climates of a model employing normal and twice the normal concentrations 
of atmospheric C02. From the results of such experiments the climatic 
changes expected at other (and generally lower) levels of CO^ are 
then found by simple interpolation, usually logarithmic. This approach 
overlooks the facts that all the elements of the climate system do not 
interact either at the same rate or in the same places, especially 
insofar as they involve the oceans, and that the C0 2 concentration 
will not "wait" at any level for the climate to reach an equilibrium. 
In this connection, model experiments treating an exponential growth of 
C0 2 , and hence a linear growth of its effects, may be particularly 
helpful. The information necessary for a realistic schedule of changes, 
including their seasonal and geographical distribution, in a variety of 
climatic parameters that are expected to occur as the CO 2 concentra- 



332 

tion reaches progressively higher levels, can be provided only by 
comprehensive general circulation model (GCM) simulations; such studies 
are important in the design of monitoring strategies. 



5.3.2.2 Initial Selection of Parameters 

A program for monitoring and detection of CO 2 effects can be developed 
on the basis of our expectations of the effects of increasing C0 2 , 
estimates of the accompanying climatic noise , and a schedule of the 
expected responses to increased C0 2 . Existing knowledge is adequate 
to formulate an initial monitoring strategy that can be revised as our 
understanding improves. The first step should be to identify the 
climatic parameters whose responses to increased C0 2r individually or 
in combinations, are likely to be significant. Atmospheric model 
studies suggest that likely candidates are the tropospheric and surface 
air temperature (which should rise) , sea temperature (which should 
rise), stratospheric temperature (which should fall), and atmospheric 
water vapor or specific humidity (which should increase). In addition, 
the downward flux of infrared radiation at the surface should increase, 
while at the top of the atmosphere the spectral distribution of out- 
going infrared radiation should shift with respect to the primary C0 2 
emission bands. In the ocean and cryosphere, the amount of snow and 
ice in polar regions should decrease over the long term and the global 
sea level should rise. 

In addition to large-scale responses, GCM climate simulations are 
expected to indicate the geographical and seasonal characteristics of 
such changes, and their evolution over time, all of which may provide 
additional indications. For example, the expected polar warming may 
occur principally in winter because of the increase of heat conduction 
through thinner sea ice, while soil moisture in midlatitudes may be 
reduced in summer (cf., Chapter 3). Other changes whose characteristics 
are yet to be definitively shown by climate model simulations may occur 
in the meridional circulation and hydrologic cycle. 

The statistical significance of changes in individual variables can 
be judged against appropriate measures of their natural variability. 
Assessment of the significance of changes in some composite index could 
be judged similarly. 



5.3.2.3 Revision and Application of a Monitoring Strategy 

A multiple-effects monitoring strategy for CO 2 -induced climatic 
change can be implemented even though all the information required for 
its rigorous design is not available. Using a combination of model 
simulations and observational data, at least partial information on the 
expected changes and levels of variability can be assembled. As further 
information becomes available from both new observations and new model 
simulations, additional variables can be included and a revised schedule 
or timetable of the effects expected over future years can be prepared. 



333 

The interpretation of the climatic record should also include an 
assessment of the possible effects of changes in other external factors 
in the climate system, apart from C0 2 and apart from internally gen- 
erated noise. Such possibly competing factors include the large-scale 
injection of aerosols into the atmosphere by volcanic eruptions and 
possible variations of the solar constant on both interannual and 
decadal time scales. As with CO 2 , our best hope for a knowledge of 
the climatic effects of these events rests with the use of a hierarchy 
of models, including comprehensive GCMs. Some experiments have sug- 
gested, for example, that the annual average (equilibrium) tropospheric 
warming resulting from an increased solar constant resembles in some 
respects that found with increased CO 2 , although the transient 
response might differ in some respects (Schneider and Thompson, 1981) . 
Since such factors have been used in conjunction with changes in CO 2 
in simple climate models to simulate the historical variations of the 
northern hemispheric surface temperature (Hansen et al., 1981), it is 
important that simulations be made with comprehensive GCMs in which the 
significance of any climatic changes due to changes in solar radiation 
can be determined and, if possible, differentiated from those due to 
other factors. This information will help us to interpret monitored 
climate records in terms of signals due to variations in incoming 
radiation, aerosol loading, or greenhouse gas concentrations and 
residual changes that may contain a signal attributable to increased 
C0 2 . 

5*3.3 Candidate Parameters for Monitoring 

Previous sections have discussed the problems of identifying C0 2 - 
induced warming from present data and showed that there are weaknesses 
in the interpretations, caused by the inadequacy of the existing data 
sets, the methods of analysis used, and the underlying theory. Data 
employed, except in recent times, have rarely been collected with 
long-term monitoring of C0 2 effects in mind, so that development of a 
coherent and integrated monitoring strategy becomes essential at this 
time. Monitoring of C0 2 effects is a challenging task if the number 
of parameters to be monitored is to be kept low, in order to reduce 
costs, and if at the same time unambiguous cause-effect relationships 
are to be established. 

The following criteria were chosen to assess the suitability of a 
number of parameters for long-term monitoring: 

1 Sensitivity. How does the effect exerted on climate by the 
variable or the changes experienced by the variable on decadal time 
scales compare with that associated with corresponding changes in CO 2 ? 

2. Response characteristics. Are changes likely to be rapid enough 
to be detectable in a few decades? 

3. Signal-to-noise ratio. How large are the relevant changes in 
relation to the variability due to measurement errors and causes not 
accounted for? 



334 

TABLE 5.7 Primary Parameters for Monitoring the Causes and Effects of 
Climate Change 



A* Primary parameters for monitoring the causes of climate change 
Carbon dioxide 
Stratospheric aerosols 
Solar radiance 
Other "greenhouse" gases and ozone 

Nitrous oxide (N 2 O) 

Methane (CH 4 ) 

Chlorocarbons (CFCl 3r CP 2 Cl 2 f for example) 

Stratospheric ozone 

Tropospheric ozone 

B. Primary parameters for monitoring the effects of climate change 
Atmosphere 

Global temperature 

Mean surface air temperature 
Tropospheric temperature distribution 
Stratospheric temperature distribution 
Radiation 

Upward terrestrial and reflected solar radiation at the top 

of the atmosphere 
Cloud and water vapor 

Precipi table water content of the atmosphere 
Equivalent emission temperature (cloudiness) 
Cryosphere 

Sea ice cover 
Snow cover 

Ice cap mass balance changes 
Oceans 

Sea level 

Sea temperature 



4. Past data base. Are data on the past behavior of the variable 
adequate for determining both a base level and its natural variability? 

5. Spatial coverage and resolution of required measurements. 
6 * Required frequency of measurements. 

7. Feasibility of technical systems. Can we make the required 
measurements? 

Cost is another important consider at ion , but meaningful assessments 
of technical feasibility and estimates of costs were beyond the 
capabilities of the present panel. As has been pointed out in other 
sections of this report, the detection of climate change is largely a 
signal-to-noise ratio problem, and it is therefore important to compile 
comprehensive data sets and to apply the best statistical techniques 
available to the data. The technical feasibility of making the measure- 



335 

ments and the associated cost f however, will put restrictions on any 
monitoring strategy that can be put into effect. However, a number of 
primary parameters for monitoring have been tentatively selected (Table 
5.7) , based on the above criteria. They are organized under the head- 
ings of "causes" of climatic change and "effects" of C0 2 . In the 
latter category, parameters to be monitored have been grouped under the 
headings of atmosphere, cryosphere, and oceans. This is not a 
comprehensive list of candidate parameters but includes only those that 
have high aggregate ratings in the seven criteria listed above. 



5.3.3.1 Causal Factors 

External factors that have been suggested as influencing the climate 
have time scales, i.e., periodicities or e-folding times, ranging from 
less than 1 to more than 10 9 years. Focus on the climatic effects of 
anthropogenic CO2 emissions allows us to limit our attention to those 
that have time scales of one to several hundred years. Within this 
time range, the external factors that may influence the climate include 
the composition of the atmosphere, volcanic activity, land surface 
modification, and solar variations. Table 5.8 lists trace gases and 
aerosols that affect atmospheric composition and that are influenced by 
man's activities. Table 5.9 lists the trace gases with potentially 
important radiative, climatic, or chemical effects. We further limit 
our consideration to external factors that might induce hemispheric or 
global temperature changes of magnitude 0.1 K or greater over about a 
century and to a lesser extent according to feasibility of monitoring. 
According to these criteria, monitoring of the following factors is 
particularly important: 

1. Carbon dioxide 

2. Solar radiance 

3. Stratospheric aerosols 

4. Other greenhouse gases* and ozone 

(a) Nitrous oxide 

(b) Methane 

(c) Chlorocarbons 

(d) Stratospheric ozone 

(e) Tropospheric ozone 

Although ozone is not strictly an external factor, its chemistry is 
affected by anthropogenic emissions of a number of gases (e.g. , nitre 
gen oxides and chlorocarbons) ; by monitoring ozone directly / * a 
dependence on knowledge of the details of the chemistry ^ o 
sulfur (DCS and SC>2, for example) and volcanic emissions both 



Kf **.** WtJb \WWW dAAVk fc^^^V , +f^r^ ^ -- ^- - - 

contribute to stratospheric aerosol. 

Several other potential external influences have --"- albedo , 
at this stage. The possible effects of modifications in surface albedo, 

~~*This list will no doubt be revised as research continues. See 
Machta (this volume, Chapter 4, Section 4.3). 



336 
TABLE 5.8 Principal Anthropogenic Sources of Trace Gases and Aerosols 



Anthropogenic Source Comments 



Gas 
C0 2 
CO 

Hydrocarbons 

C 2 H 4' for example 
Chlorocarbons 

CH A 



NO, NO 2 



Sulfur Compounds 
OCS, CS 2 



S0 2 
Ozone 



Aerosols 



Fossil fuel combustion 

Internal combustion 

engines 
Internal combustion 

engines 
Refrigerants, 

solvents, propellants 
Internal combustion 

engines, industry, 

change in land use 
Combustion, 

fertilizer manufacture 
Internal combustion 

engines, aircraft 



Fossil fuel conversion 



Possibly large bio spheric 
component 



Those of concern entirely 

man-made 
Large component from 

biological activity 

Large natural component 
from biological activity 

High-flying aircraft are 
an upper-tropospheric and 
lower-stratospheric source 



Volcanoes are an 
intermittent 
source of sulfur 
Combustion 

Anthropogenic contribution 
is from chemical reaction 
of other trace gases 



Sulfate 



Silicate or 

car bon-conta in ing 



Conversion from S0 2 
and other sulfur- 
bearing compounds 

Combustion, 
soil erosion 



Most important for 

stratospheric aerosols 

Diesel engines especially, 
closely tied to land use 



roughness, and surface thermal characteristics by desertification, 
urbanization, and extension of agriculture (e.g., Charney, 1975; Sagan 
et al., 1979; Potter et al, 1975, 1980) are not included because 
effects of such changes appear to be primarily regional and because of 
the difficulty of developing a data base. Tropospheric aerosols are 
not included because there are large regional variations in aerosol 
amount, composition, and characteristics, so that an accurate monitor- 
ing network would require many stations as well as detailed analysis of 
the samples. These neglected effects contribute, of course, to the 



337 



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338 

unexplained variance in the climatic record. Clearly, continuing 
research on the possible influences on climate and on the means for 
their measurement is essential. 

The following subsections provide more detailed discussion for those 
external parameters listed above and present monitoring requirements. 
In addition to recommending improved monitoring of some of the f actor s f 
in several cases it will be essential to continue the records of 
surrogate indicators that have been developed in order to be able to 
calibrate these methods and extend the data bases back in time. 

5.3.3.1.1 Carbon Dioxide 

Sensitivity. Numerical model studies indicate that doubling 
atmospheric carbon dioxide concentrations would increase global average 
temperatures (AT d ) by 3 + 1.5C (Climate Research Board, 1979; 
C0 2 /Climate Review Panel, 1982) . Changes in radiative flux are 
approximately logarithmically related to C0 2 concentration. Although 
water vapor and albedo feedback mechanisms are a function of global 
mean temperature, for a moderate warming, temperature change (AT) is 
nearly linearly proportional to radiative flux, so that 




where AT d is the expected temperature change for doubling, and 
[CO 2 ] and [C0 2 ] are the concentrations for the base period and 
the current period, respectively. 

Rate of Change. Since 1958, the annual increase in the atmospheric 
concentration of C0 2 has ranged between 0.6 and 2.2 ppmv (Machta, 
this volume, Chapter 3, Section 3.4). The concentration in 1982 was 
slightly above 340 ppmv. The concentration before the industrial 
revolution probably was in the range 250-290 ppmv. Releases of C0 2 
to the atmosphere result from both fossil fuel emissions and changing 
land use. The mean growth rate of fossil fuel C0 2 production over 
the period 1861-1980 was 3.5% per year (Elliott, 1982). In 1980 total 
CO 2 emissions from fossil fuels are estimated to have been about 5.2 
x 10 9 tons of carbon (Mar land and Rotty, 1982) . Uncertainty asso- 
ciated with calculations of fossil fuel CO 2 emissions is estimated to 
be 10-13.5%. C0 2 emissions from cement manufacture may add an 
additional 2% to the estimate of fossil fuel emissions. The history of 
biospheric emissions is poorly known and controversial. Global net 
release of carbon to the atmosphere from the biosphere since 1860 has 
been estimated to be as high as 180 x 10 9 tons of carbon. The release 
from biospheric sources in 1980 was given as 1.8 x 10 9 tons of carbon 
according to the FAO Production Yearbook, the most comprehensive and 
apparently detailed data source, and has been estimated to be as high 
as 4.7 x 10 9 tons of carbon according to other sources (see Woodwell, 
this volume, Chapter 3, Section 3.3). Regrowth and stimulation of 
photosynthesis from increased atmospheric CO 2 may have compensated 
for some of these emissions (see Machta, this volume, Chapter 3, 



339 

Section 3.4). Nordhaus and Yohe (this volume, Chapter 2, Section 2.1} 
project that the atmospheric CO 2 concentration will most likely 
increase by an average of 0.3% per year up to the year 2000 , reaching a 
concentration at that time of 367 ppmv* 

Signal-to-Noise Ratio. The C0 2 concentration in a single sample can 
be measured to within 0.1 ppm, but at most stations in the global net- 
work the accuracy is about 1 ppm. The seasonal cycle and annual 
increase of atmospheric C0 2 can be easily detected at remote locations 
where effects of local biospheric f anthropogenic, and geologic sources 
are avoidable. The atmosphere, averaged on a zonal and annual basis, 
appears to be within 4-6 ppm of being well mixed, and gradients of this 
magnitude can be explained by longer times for transport from oceanic 
or biologic sources and sinks (see Machta, this volume, Chapter 3, 
Section 3.4) . 

Adequacy and Availability of Data Base. Continuous, accurate observa- 
tions of C0 2 began at Mauna Loa Observatory in 1958. Additional 
research and measurements are needed to reconstruct past concentrations. 
Before 1958, most measurements are of uncertain accuracy and may not be 
representative of the global average concentration. For purposes of 
detection of climate change, the present network provides adequate 
measures of global average concentrations, and data are readily 
available . 

Efforts are under way to estimate past CO 2 concentrations by, for 
example, analysis of solar spectral measurements, carbon isotopic ratios 
in tree rings, reconstructed pCO 2 in former oceanic surface waters, 
and C0 2 concentration in air bubbles trapped in ice. Efforts to 
reconcile the results of these various approaches, including the effects 
of local station bias and different temporal characteristics, are 
needed. 

Spatial Coverage and Resolution of Additional Measurements Required. 
The present global network provides adequate coverage and resolution of 
C0 2 concentration for purposes of detection of climatic changes. 

Frequency of Measurements Required. For purposes of studying the 
climatic effect, the record of monthly average C0 2 concentration now 
being made is sufficient. Climate model studies of the climatic 
effects of increasing C0 2 concentrations are most unlikely to use 
more frequent data than annual average concentration. 

Feasibility and/or Existence of Technical Systems; Continuity. Mea- 
surements made by the present global network of surface stations are 
satisfactory for determining the trend of C0 2 concentrations. 
Although satellite measurement of atmospheric CO 2 would provide 
improved global coverage, it is not evident that sufficient accuracy 
could be achieved to determine the inter annual variations in the trend 
or in the range of the seasonal cycle* 



340 
5.3.3.1.2 Stratospheric Aerosols 

Sensitivity, Stratospheric aerosols affect climate through their net 
effects on the components of the Earth's radiation budget. The deli- 
cate balance that exists between the solar and thermal components of 
the radiation budget can be altered by the range of possible strato- 
spheric aerosol (or cirrus cloud) scattering and absorption properties 
such that either a net cooling or warming of the lower atmosphere may 
result. Radiative transfer calculations (e.g., Pollack et al., 1976b) 
indicate that the sign and magnitude of the temperature change depend 
critically on the composition (refractive indices), size distribution, 
scattering phase function, and optical depth of the stratospheric 
aerosols. Surface-temperature changes associated with potential 
radiative perturbations of climate were calculated by various research 
groups. For example, Hansen et al. (1981), using a simplified one- 
dimensional model, calculated that a persistent increase in strato- 
spheric (sulfuric acid) aerosol optical depth of 0.2 (representative of 
a large volcanic event) would produce a cooling of 1.9C in the Earth's 
surface temperature. In reality, however, effects over land and sea 
might be quite different. 

Natural volcanic eruptions throughout history have long been thought 
to constitute the primary source of aerosols in the stratosphere 
through the direct injection of large quantities of gases and ash. 
Although the Mount St. Helens eruption was powerful and injected large 
quantities of material into the stratosphere (-0.5 km 3 ), its 
climatic impact was insignificant (Robock, 1981, 1983). Climatic 
effects of volcanic eruptions, in general, depend on the chemical 
composition of the aerosols injected into the stratosphere and, in 
particular, on the amount of sulfur contained. The climate effects are 
n t a simple function of the volume of solid material ejected or the 
magnitude of eruption. 

Rate of Change. Several empirical and statistical studies offer a 
history of volcanic impact on climate. A discussion of climatological 
evidence from past volcanoes is given in the recent report of the NASA 
Workshop on the Mount St. Helens Eruption of 1980 (Newell and Deepak, 
1982) . Lamb (1970) proposed an empirical dust veil index (DVI) in 
correlating atmospheric optical depth perturbations of the atmosphere 
with volcanic eruptions over the past century. Thus, the historical 
record of volcanic eruptions must account for composition factors in 
assessing climatic effects. This holds for future extrapolations. 
There is no strong basis for projecting aerosol perturbations or change 
of composition. No clear trend or pattern emerges from the historical 
record. 

Signal- to-Noise Ratio, Satellite measurements of peak background 
(pre-eruption) stratospheric infrared extinction (1.0 ym) are 
typically on the order of 10~ 4 km" 1 (McCormick et al. , 1981) . The 
corresponding optical depth between the tropopause and 30-km altitude 
is approximately 10" 3 . Immediately following the Mount St. Helens 
eruption, local extinction values as high as 10" 1 km" 1 (optical 



341 

depths near 1) were observed , although the average stratospheric 
optical depth was typically 10" . Volcanic eruptions f which are 
believed to have had an impact on climate, have produced stratospheric 
optical depths of about T - 10"^ for several years following 
the eruptions. 



Adequacy and Availability of Data Base. Volcanic eruptions have been 
related to climatic changes since a suggestion by Benjamin Franklin in 
1784 (cf. Pollack et al. r 1976a) . Extended inventories of volcanic 
eruptions have been prepared by Lamb (1970) and Simkin et al. (1981) * 
Pollack et al. (1976a) have assembled and reviewed data on transmis- 
sivity. Actinometric data, which may also include the effects of 
tropospheric aerosols, have been assembled by Pivovarova (1968, 1977) 
and Bryson and Goodman (1980) . Hammer (1977) suggests that acidity in 
a Greenland ice core may be a measure of northern hemisphere volcanic 
activity. In recent years lidar data have become available, but time 
histories exist over only limited parts of the globe for about 10 years. 

Observations of stratospheric aerosol extinction have been made by 
the Stratospheric Aerosol Measurement II (SAM II) instrument aboard the 
Nimbus- 7 research satellite (since October 1978) and by the Strato- 
spheric Aerosol and Gas Experiment (SAGE) aboard the Applications 
Explorer Mission 2 (AEM-2) (February 1979-March 1982) . A follow-on 
SAGE mission, SAGE II, is planned for launch in 1984 in conjunction 
with the Earth Radiation Budget Experiment (ERBE) . Aerosol information 
may also be inferred from operational Advanced Very High Resolution 
Radiometer (AVHRR) data from National Oceanic and Atmospheric Adminis- 
tration (NOAA) satellites. 

Stratospheric aerosol data products are routinely archived at the 
National Space Science Data Center (NSSDC) at the National Aeronautics 
and Space Administration's (NASA) Goddard Space Flight Center, 
Greenbelt, Maryland. 

Spatial Coverage and Resolution of Additional Measurements Required. 
Future high-latitude coverage beyond SAM II will be needed for inves- 
tigating the importance of polar stratospheric clouds (PSCs) on climate. 
SAGE covered the geographical regions between 72 N and 72 S latitude, 
while SAM II observes both polar regions. Planned SAGE II measurements 
will provide continuing aerosol extinction measurements at low and 
midlatitudes through the mid-1980s. No corresponding measurements are 
planned for the polar regions. Remote measurements must be accompanied 
by in situ measurements of aerosol radiation properties and size dis- 
tribution, as part of a coordinated ground-truth program. For example, 
lidars and aircraft flights are needed to calibrate the satellite mea- 
surements. In particular, additional lidars are needed in the southern 
hemisphere. Comprehensive data, including latitudinal variation, are 
needed in aerosol impact model studies. 

Frequency of Measurement Required. Measurements are needed to establish 
the monthly variation of stratospheric aerosols. 

Current and planned satellite observational programs are believed to 
be adequate for establishing the seasonal and inter annual variability 



342 

of the present background stratosphere. Provision must be made to 
respond to major volcanic eruptions as they occur and to continue 
observations beyond the lifetime of current programs. 

Feasibility and/or Existence of Technical Systems; Continuity. Existing 
satellite systems supplemented by a few key lidar systems for calibra- 
tion purposes are adequate. 

5.3.3.1.3 Solar Radiance 

Sensitivity. Solar radiation is the fundamental energy source that 
drives the motions of the atmosphere and oceans. The associated storage 
and transport of heat and mass then establish the Earth's temperature 
and climate on regional and global scales. Temporal variations in the 
sun's output have long been thought to be a major cause of recorded 
climate change. Although a great number of researchers have sought 
clear evidence in linking solar variations to specific weather and 
climate responses, the results have thus far been inconclusive. A 
recent report by the Geophysics Study Committee (1982) concludes that 
the role of the sun in producing global circulation/ climatic zones, 
seasonal changes, and the recurrence of periods of glaciation is well 
recognized, but intrinsic solar variability is neither implied nor 
required to account for these phenomena. The simple changes in 
insolation responsible for these effects are predictable, far in 
advance, on the basis of known parameters of the orbit, figure, and 
motions of the Earth. 

Although numerous mechanisms exist that can cause climate change, it 
is important, nevertheless, to understand to what extent the basic 
solar forcing of the climate system is changing. Sensitivity studies 
reported by Hansen et al. (1981) suggest a 0.5C global average surface 
temperature rise associated with a hypothetical 0.3% increase in the 
solar constant. Systematic changes of only 0.5% per century could 
explain the entire range of past climate from tropical to ice-age 
conditions (Eddy, 1977) . 

Rate of Change. The integral of the broad spectrum of solar electro- 
magnetic radiation incident on the Earth has historically been termed 
the solar constant, as a consequence of the lack of evidence to the 
contrary. Until recently, detailed information on short-term natural 
variability in the solar constant was restricted mainly by the lack of 
adequate instrumentation and by the variability in atmospheric trans- 
mission, which limits the accuracy of surface-based observations of the 
sun. 

Since late 1978, independent accurate measurements of solar radiance 
by two NASA research satellites have confirmed that the solar constant 
is indeed variable. Both the Solar Maximum Mission (SMM) and Nimbus 7 
spacecraft carry advanced active cavity radiometers that overcome the 
basic deficiencies in prior measurements. SMM results reported by 
Willson et al. (1981) yield a mean solar radiance at 1 AU of 1368 W 
m" 2 with an absolute uncertainty of less than +0.5%. Several observed 
large decreases (dips) in radiance of up to 0.2%, lasting about 1 week, 



343 

are highly correlated with the development of sunspot groups whose 
Delow-average temperatures reduce the total output of solar radiation. 
Solar faculae, with above-average temperatures and, therefore, radia- 
tive output higher than normal, cause a much smaller variability 
appearing as a radiative excess. The facular effects correlate with 
Lrradiance peaks before and after some observed dips. A measured 0.05% 

per year downward trend in the 4-year record may be related to the ,- * ' 

general pattern of solar activity variation over an 11-year cycle. * 

3iqnal~to-Noise Ratio. The solar radiance monitor carried aboard the 1 
3MM spacecraft consists of a combination of three independent elec- -I 
trically self-calibrated cavity pyrheliometers having nearly uniform s 
wavelength sensitivity from the far-ultraviolet through the far- 1 
Infrared regions. This sensor combination has provided daily obser- 
vations of the sun with an estimated measurement precision of less than i 
D.O05% since launch. Such precision should be sufficient to detect I 
solar variability of a magnitude able to affect climate. 

\aeguacy and Availability of Data Base. Routine satellite measurements : 

3f solar radiance have been made since October 1978. This measurement 
capability, if sustained, is believed adequate for meeting current 
slimate requirements for broadband solar radiance monitoring. 

Spatial Coverage and Resolution of Additional Measurements Required. 

Current measurements should be continued to provide a data base 

encompassing at least one 11-year solar cycle. I 

Frequency of Measurements Required. The wide range of natural 
variability found in the current data set supports the need for daily 
measurements over the next few years. Beyond then, less frequent 
(monthly?) measurements may be adequate. 

Feasibility and/or Existence of Technical Systems? Continuity. Although 
current technology can meet the scientific requirements for broadband 
solar radiance monitoring, no future satellite missions carrying these 
instruments have yet been approved. The proposed Upper Atmosphere 
Research Satellite (UARS) would have this capability. 

5.3.3.1.4 Nitrous Oxide (N 2 0) 

Sensitivity. Donner and Ramanathan (1980) and Wang et al. (1976) | 

calculated a surface warming of 0.3 K and 0.4 K, respectively, for a | 

doubling of the N 2 O concentration. A doubling of N 2 concentra- | 

tions would also decrease the total ozone column by as much as 15% I 

(World Meteorological Organization, 1981) through the catalytic ^ 

reactions of N 2 0-produced nitrogen oxides with ozone; the change in . 

3 concentration would also have climatic effects. | 

Rate of Change Current tropospheric N 2 concentrations are approxi- ; | 

mately 300 ppbv. N 2 levels appear to be approximately 0.8 ppt>v 
higher in the northern hemisphere than in the southern hemisphere. 



j 



344 

Atmospheric measurements (Weiss, 1981) indicate that tropospheric 
concentrations of nitrous oxide (N 2 O) have been increasing at a rate 
of approximately 0.2% per year since the first measurements were made 
in 1963. Weiss suggests that a substantial fraction of this increase 
may be explained by combustion of fossil fuels. Nitrogen fertilizers 
may also be an important source of N 2 O. It would be desirable to 
find means for determining the pre- industrial N 2 concentrations in 
the atmosphere. 

The N 2 O concentration decreases with altitude in the stratosphere. 
The primary sink for N 2 is photodissociation in the stratosphere. 
The primary source of nitrogen oxides in the present stratosphere is 
the reaction of N 2 with excited oxygen atoms. 

Signal~to~Noise Ratio. The N 2 concentration at the surface can now 
be measured with a relative precision of less than 0.5% (Weiss / 1981) 
and to an absolute accuracy of 3%. Because of its long lifetime / N 2 
in the troposphere is well mixed/ to within 5 ppbv/ i.e./ 1.7% of its 
mean (World Meteorological Organization/ 1981) . This range is explain- 
able in terms of dynamics and variations in strong local sources and 
sinks (Levy et al./ 1979). 

Adequacy and Availability of Data Base. Measurements of N 2 O exist 
since 1961/ with more extensive measurements beginning in 1976. 
Several groups have undertaken programs to monitor tropospheric N 2 
(Weiss/ 1981; Pierotti and Rasmussen/ 1977/ 1978). Weiss (1981) 
reports measurements at three NOAA Geophysical Monitoring for Climatic 
Change (GMCC) sites (Mauna Loa/ Point Barrow/ and South Pole) plus ship 
data. 

Spatial Coverage and Resolution of Additional Measurements Required. 
The surface sources and sinks of N 2 are still not well understood. 
Additional research is needed, and continued surface monitoring should 
be supported. 

Frequency of Measurements Required. A monthly record at several sites 
in each hemisphere is desirable. 

Feasibility and/or Existence of Technical Systems; Continuity. Current 
ground-based observations should continue in order to monitor global 
concentrations, and satellite measurements of N 2 are likely to 
become more available within the next decade. The latter may be better 
able to determine tropospheric and stratospheric variations in N 2 0. 

5.3.3.1.5 Methane (CH 4 ) 

Sensitivity. For a doubling of the present CH 4 concentrations, Wang 
et al. (1976) estimated a surface warming of 0.2-0.4 K/ depending on 
model assumptions. Dormer and Ramanathan (1980) and Lac is et al. (1981) 
calculated a warming in surface temperature of 0.3 K for the same 
change in CH4 concentrations. Changes in methane concentration may 
also influence the global ozone distribution due to the reactivity of 
methane with hydroxyl radicals and other trace gases. 



345 

Rate of Change. Major sources of methane (CH 4 ) are believed to be 
mining and production of fossil fuels; anaerobic fermentation of organic 
material due to microbial action in rice paddies, swamps and marshes, 
tropical rain forests, and tundra; and enteric fermentation in mammals 
and the activity of termites. The primary atmospheric sink is by reac- 
tion with hydroxyl radical (OH) in the troposphere and stratosphere. 
Methane degradation is an important source of atmospheric carbon 
monoxide. Water vapor and odd hydrogen species (OH, HO 2 ) are impor- 
tant products of CH 4 oxidation in the stratosphere. The atmospheric 
lifetime of methane is approximately 7 years. Current tropospheric 
concentrations are about 1.7 ppmv. Methane concentrations have 
increased 1-2% per year since careful observations intended to determine 
presence of a trend were started in 1978 (Rasmussen and Khalil, 1981; 
see Machta, this volume, Chapter 4, Section 4.3). 

Signal-to-Noise Ratio. At this time it is difficult to assess the 
extent to which the recorded increase represents a short-term fluc- 
tuation in the methane cycle or a long-term trend. Historical studies 
of methane concentrations are desirable. 

Methane can be measured with a precision of 0.01 ppmv and an accuracy 
within a few percent. CH 4 is reasonably well mixed in the troposphere 
but decreases in concentration with altitude in the stratosphere owing 
to reaction with OH and other radical species. More CH 4 is found in 
the northern hemisphere than in the southern hemisphere. 

Adequacy and Availability of Data Base. Regular measurements of CH 4 
have been made for only a few years. 

Spatial Coverage and Resolution of Additional Measurements Required. A 
few sites in each hemisphere should be adequate, if well chosen. The 
existing programs may be sufficient, although additional measurements 
may be needed for a better understanding of surface sources and sinks 
of CH 4 and variations with altitude. 

Frequency of Measurements Required. A monthly record at several sites 
in each hemisphere is desirable. 

Feasibility and/or Existence of Technical Systems; Continuity. Ground- 
based measurements may need to be expanded to improve monitoring of 
globally averaged tropospheric concentrations. Satellite measurements 
may become available within a decade. 

5.3.3.1.6 Chlorocarbons (e.g., CFC1 3 , CF 2 C1 2 ) 

Sensitivity. A number of chlorocarbons and chlorof luorocarbons have 
strong IR bands (Ramanathan, 1975; Wang et al., 1976). Ramanathan 
(1975) estimated that increasing the concentrations of both CFCl-j and 
CP 2 C1 2 to 2 ppbv could raise the surface temperature by 0.9 K. For 
the same chlorocarbon abundances, Lacis et al. (1981) calculated a 
surface temperature change of 0.65 K. Other chlorocarbons with known 
absorption features in the same region are CC1 4 , CHC1 3 , CH 2 C1 2 , 



346 

and CH 3 C1. Chlorocarbon emissions are also of concern because they 
dissociate in the stratosphere, and the resulting ClO^ species might 
significantly affect stratospheric ozone concentration through ozone- 
destroying catalytic reactions. 

Rate of Change. Large quantities of these industrially produced chemi- 
cals are made for a variety of uses, such as solvents, refrigerants, 
and spray-can propellants. The chlorocarbons of primary current concern 
are CFC1 3 (also referred to as CPC-11) and CFoC^ (CFC-12) because 
of large amounts produced, long atmospheric lifetimes, and increasing 
stratospheric and tropospheric concentrations. Concentrations of these 
two chlorocarbons continue to increase by approximately 10% per year, 
although rates of emission have not increased significantly since 1976. 

The lifetime of CFC13 and CF2C12 are approximately 60 years and 
100 years, respectively. These species are well mixed in the tropo- 
sphere. Once in the stratosphere, these species eventually photodis- 
sociate or sometimes react with excited oxygen atoms. Some of the 
other chlorocarbons react with hydroxyl in the troposphere. These 
species have much shorter tropospheric lifetimes. 

Signal-to-Noise Ratio. CFC1 3 and CF 2 Cl2 are well mixed in the 
troposphere with mixing ratios of approximately 190 and 320 pptv, 
respectively. Estimated precision for CFC13 and CF 2 Cl2 
measurements are 2*5% (World Meteorological Organization, 1981) . Ten 
percent is the estimated accuracy for measurements of CFC1 3 and 
CF2C1 2 , with the limit of detection being about 1 pptv. Approxi- 
mately 10% larger concentrations of CFC1 3 and CF 2 c l2 ex i st i n tlle 
northern hemisphere than in the southern hemisphere. Errors are larger 
for other chlorocarbons. 

Adequacy and Availability of Data Base. The present programs sponsored 
by the federal government and by the Chemical Manufacturers Association, 
if continued, are probably adequate to monitor the increase in the most 
important of these species. Additional data would be useful for such 
other chlorocarbons as CH 3 CC1 3 , CFC1 2 , CF 2 C1, and CC1 4 . 

Spatial Coverage and Resolution of Additional Measurements Required. 
Because radiatively important chlorocarbons are long-lived species and 
relatively well mixed in the troposphere, measurements at only several 
sites in each hemisphere are required. This can be accomplished by 
assuring that the record from sites now sponsored for research purposes 
(or similarly located sites) is continued for monitoring purposes. 

Frequency of Measurements Required. A monthly record of the con- 
centration of a number of chlorocarbons at several sites in each 
hemisphere is desirable. Because of the accuracy required to detect 
changes in chlorocarbons with time, additional sites would have to be 
chosen carefully, so that long records can be developed. 

Feasibility and/or Existence of Technical Systems Required; Continuity. 
Present ground-based measurements are probably adequate for determina- 



347 

tion of global concentrations. Satellite measurements may come 
available within the next decade. ^ments may come 

5.3.3.1.7 Stratospheric Ozone 

Sensitivity. Stratospheric ozone can affect climate through its 
influence on dynamic and radiative coupling mechanisms between the 
stratosphere and troposphere. The absorption of solar radiation by 
stratospheric ozone is primarily responsible for the increase in 
stratospheric temperatures with altitude and is thus linked to the 
aynamics of the stratosphere. Bates (1977) and Geller and Alpert 

(1980) examined the effect on tropospheric planetary waves from changes 
in the zonal mean wind and temperature structure in the stratosphere. 
Bates (1977) found that dramatic changes in the northward flux of 
sensible heat resulted from changes in stratospheric structure. Geller 
and Alpert (1980) found that the stratospheric structure had to be 
altered below about 35 km before any significant changes in the struc- 
ture of tropospheric planetary waves resulted. Many uncertainties 
remain in determining the effect of changes in stratospheric ozone on 
tropospheric dynamics. However, the modeling studies described above 
suggest that planetary wave coupling may link tropospheric weather and 
climate to changes in the stratosphere. 

A reduction in stratospheric ozone can modify the surface tem- 
perature through two competing radiative processes (solar and long 
wave) . With less ozone f more solar radiation is transmitted through 
the stratosphere, thereby enhancing solar heating of the troposphere 
and Earth's surface. On the other hand, the reduced absorption of 
solar radiation in the stratosphere cools the stratosphere, thereby 
reducing long-wave emission from the stratosphere downward into the 
troposphere by all long-wave emitting species. The changes in solar 
and long-wave fluxes have opposing effects on surface temperature. 
Ramanathan et al. (1976) calculated a surface cooling of 0.1 K for a 
10% uniform reduction in stratospheric ozone. However, Ramanathan 

(1980) has shown that changes in the vertical ozone distribution can 
have a significant effect on surface temperature, even if the total 
ozone column does not change. Since the change in ozone concentration 
with altitude differs, depending on the source of this perturbation 

(e.g., CFCs, NC^) , the change in surface temperature may differ 
Depending on the perturbation even if the change in total ozone is the 
same (irrespective of the radiative effect of the perturbing species) . 

Rate of Change. Several analyses have been made of measurements of 
total ozone from 37 Dobson stations. The studies by Bloomfield et al. 
(1981), St. John et al. (1981), and Reinsel (1981) give the following 
95% confidence intervals for global increase in total ozone in the 
10-year period 1970-1979: 

Bloomfield et al. (1.7 2.0)% 
St. John et al. (1.1 I- 2 )* 
Reinsel (0.49 1.3)% 



348 

Total ozone variations differed significantly with geographical 
location of the instrument* Total ozone increased at some stations and 
decreased at others. 

Satellite measurements of ozone concentration in the upper strato- 
sphere between 1970 and 1979 indicated a reduction of up to 0.46%/yr at 
altitudes between 33 and 43 km (Heath and Schle singer, 1982). The 
satellite data are not consistent concerning whether ozone increased or 
decreased at higher altitudes. Recent analyses of surface-based Umkehr 
data are consistent with a decrease in upper stratospheric ozone of 
0.3-0.4%/yr during the 1970s (Reinsel et al., 1983). 

Signal- to-Noise Ratio. There is a great deal of variation in the ozone 
record. Ozone concentrations fluctuate on a variety of spatial and 
temporal scales owing to natural causes; these fluctuations tend to 
mask possible systematic changes due to manmade perturbations. Observed 
ozone changes averaged over the northern hemisphere and the world 
suggest that a time interval of approximately 10 years is the shortest 
period that is meaningful for calculating ozone trends. Ozone varies 
with the 11-year solar cycle r and there is also a strong quasi-biennial 
oscillation (World Meteorological Organization, 1981) that affects 
trend analyses for shorter averaging times. 

Some variation is attributed to differences in instruments, in 
instrument operators/ or in adjustment/calibration procedures. Those 
Dobson stations with the longest records (several decades) are gen- 
erally considered to be the most reliable. Unfortunately, most of the 
stations began operating after 1957. The noise in the data contributes 
to the uncertainty in the trend analyses, which is considerable (as 
indicated above) . 

Satellite records are relatively short, with the first ozone 
measurements beginning in mid-1970. 

Adequacy and Availability of Data Base. An extensive global network of 
stations measuring total ozone exists. However, most of the observing 
stations are on continents and in the northern hemisphere. Satellite 
measurements providing more uniform spatial sampling could eventually 
lead to better trend measurements. Also, since only one instrument is 
used, the present intercalibration errors between surface devices are 
eliminated. 

Recent modeling studies (e.g., Wuebbles et al., 1983) suggest that 
total ozone measurements may not be a sensitive indicator of the impact 
of human activities on the global atmosphere. These studies suggest 
that monitoring of changes in the distribution of ozone with altitude 
should be more useful. However, major uncertainties exist in both the 
existing Umkehr network and satellite measurements. Only about 18 
stations currently make regular Umkehr measurements? only 3 of these 
stations are in the southern hemisphere. In addition to calibration 
drift, the Umkehr method is also subject to errors from the effect of 
dust and aerosols and the inability to make measurement under cloudy 
conditions. As with the total ozone measurement, the satellite data 
are limited by the short duration of continuous and homogeneous global 
data coverage. 



349 

The ozone data are available from several central sources. The 
Dobson data are compiled and checked prior to public release by the 
Center for Ozone Data for the World located in Toronto, Ontario, 
Canada. The data are usually available about a year after they are 
taken by the Dobson stations. 

NASA satellite data are available from the National Space Science 
Data Center at the NASA Goddard Space Flight Center, Greenbelt, 
Maryland. Data from the NOAA satellites are not available from this 
center and must be obtained directly from NOAA. It usually takes more 
than a year for the satellite data to be processed, reviewed, and 
released to the Data Center. 

Spatial Coverage and Resolution of Additional Measurements Required. 
The ozone data based on Dobson and Umkehr stations have limited geo- 
graphic coverage. Most stations are located in continental areas at 
middle latitudes in the northern hemisphere. Consequently, it is 
difficult to estimate global total ozone when there are vast areas of 
the world (mostly in the southern hemisphere) where measurements are 
not taken. The existing total ozone and ozone distribution networks 
need to be enlarged, particularly by adding stations in the southern 
hemisphere. 

The spatial coverage on ozone from satellites is very good, but the 
satellite data are limited in temporal coverage. Global satellite 
measurements of ozone began in 1970, and the longest single record of 
data is 7 years. Although a continuous ozone record is available owing 
to overlap of various satellite systems, there is a significant bias 
between systems that makes ozone trend analysis using satellite data 
difficult. 

Frequency of Measurement Required. Because of the large temporal and 
spatial variations of ozone, frequent measurements are needed. With a 
limited surface network, daily measurements of ozone should be taken 
(and usually are) . The best stations take several measurements each 
day to monitor diurnal variations and to reduce the noise in the 
measurements. 

Satellite systems monitor stratospheric ozone continuously with the 
objective of covering the entire global area at least once each day. 
Solar UV instruments usually are in polar orbits and take measurements 
at the same local time at each geographic location. IR instruments are 
capable of taking day and night measurements. Because of the operating 
cycle of satellite systems, a satellite usually has gaps in its temporal 
and spatial coverage. Daily measurements are adequate for obtaining 
monthly-mean distribution of ozone. 

Feasibility and/or Existence of Technical Systems? Continuity. The 
Dobson instruments are considered to be excellent in quality and are 
often used as a reference for comparison with satellite measurements. 
NOAA, in cooperation with Environmental Protection Agency and the 
Chemical Manufacturers Association, is currently developing a new 
Umkehr network with improved instrumentation and extensive intercali- 
bration program. Improvements are being made in satellite instrumen- 



350 

tat ion, so the data have improved in spatial resolution and in 
consistency with other ozone data. Future NOAA satellites will carry 
operational ozone-measuring instruments. 

5.3.3.1.8 Tropospheric Ozone 

Sensitivity. Although only 5-10% of the total column amount of ozone 
is in the troposphere f a uniform percentage change in tropospheric 
ozone can have about the same effect on surface temperature as the same 
percentage change in stratospheric ozone. This results from the fact 
that owing to pressure broadening of the lines in the 9.6-vm band of 
ozone/ the total long-wave opacity of tropospheric ozone is nearly the 
same as that of stratospheric ozone. The solar effect of a change in 
tropospheric ozone is different than that of stratospheric ozone in 
that both the solar and long-wave effects are in the same direction in 
this case. An increase in tropospheric ozone increases solar absorption 
plus enhances the long-wave "greenhouse" effect of ozone; both effects 
tend to increase surface temperature. Fishman et al. (1979) calculated 
that a doubling of tropospheric ozone concentrations would lead to an 
increase in surface temperature of 0.9 K. 

Rate of Change. An analysis of ozonesonde data (World Meteorological 
Organization, 1981) shows that tropospheric ozone in the layer from 2 
to 8 km increased in northern middle latitudes by about 7% during the 
last decade. An increase has also been observed in ozone in the layer 
between 8 and 16 km, but the magnitude is less than that for the 2- to 
8-km layer. The cause of this increase is not known, but it is con- 
sistent with model calculations that predict an increase in upper 
tropospheric ozone due to increased NO^ emissions from aircraft 
engines. An increase in tropospheric ozone is not observed at all 
latitudes nor at all stations in middle latitudes. Some model calcu- 
lations also suggest large photochemical production and destruction 
rates for ozone in the troposphere, implying night-day shifts in 
concentrations . 

Signal-to-Noise Ratio. The annual-mean values of the ozonesonde 
measurements in the 2- to 8-km layer have an uncertainty of +2-3%. 
Ozonesonde measurements in the troposphere have higher relative 
accuracy than in the stratosphere. As the ozonesonde reaches high 
altitudes, pump correction factors have to be applied to the 
measurements to account for reductions in pump efficiency. 

Adequacy and Availability of Data Base. Both Umkehr and ozonesonde 
data bases are limited in spatial coverage. Although Umkehr measure- 
ments are made of tropospheric ozone, the Umkehr technique is more 
accurate for ozone in the upper stratosphere. While ozonesonde 
measurements have high vertical resolution, there are only a small 
number (<20) of stations. Only one ozonesonde station is in the 
southern hemisphere. Many ozone measurements are taken near the ground 
(in the boundary layer) in urban areas to monitor pollution. To study 



351 

climatic effects, measurements are needed of the ozone distribution 
through the troposphere. 

Spatial Coverage and Resolution of Additional Measurements Required, 
The ozonesonde data give detailed information about the vertical 
profiles of ozone through the troposphere. Because these balloon 
measurements are expensive, the number of launches from each station 
per year is limited. Budget cutbacks have led to a reduction in the 
number of operating stations. Since a long data record is needed for 
trend analysis, keeping or restoring the ozonesonde network to its peak 
level is important. Although adding additional stations would be desir- 
able, maintaining what has been operational is a reasonable objective 
during this time of limited resources. 

The Umkehr network provides an independent check on the ozonesonde 
measurements, and vice versa. Because Umkehr measurements are made with 
ground-based instruments, there is a lower cost per measurement, so the 
frequency of measurements is greater than that of the ozonesonde 
network. 

Frequency of Measurement Required. Monthly mean profiles of ozone are 
needed for trend analysis. Consequently, daily measurements are 
desirable. This is feasible for the Umkehr network, but costs prohibit 
ozonesonde measurement at this frequency. At least one ozonesonde 
measurement per week would be desirable at each location. 

Feasibility and/or Existence of Technical Systems; Continuity. The 
spatial and temporal coverage of the existing ozonesonde network is 
limited, and the network should be maintained and expanded, if pos- 
sible. New, highly automated, ground-based ozone monitoring instru- 
ments have been designed, allowing greater frequency of measurements 
and less dependence on .operator skill. Because the Dobson instruments 
are well established, there will be some reluctance to replace them 
with the new instruments until the quality and reliability of the 
instrument is fully demonstrated and adequate overlap calibrations are 
available and have been carefully studied. 



5.3.3.2 Atmospheric Parameters 

The atmosphere is the part of the climate system with the least thermal 
inertia and, therefore, the part that can respond most quickly to the 
radiative effects of increasing C02 concentrations. Climate models 
have focused most attention on the changes that increasing CO 2 
concentrations will induce in the atmosphere, and it is therefore quite 
appropriate to look first to the atmosphere for any evidence of 
C02-induced climate changes. 

Since the greenhouse effect on the radiation balance of the atmo- 
sphere affects temperature directly, a first change to look for is in 
the temperature field. Global and hemispheric mean surface tempera- 
tures have been studied extensively in this regard regional tempera- 
tures cannot be considered as sufficiently representative. Arctic and 



352 

Antarctic temperatures may respond more than the global mean, but 
unfortunately the variability of polar temperatures from year to year 
is also larger, so it is unclear whether the signal-to-noise ratio will 
be less there (Kelley and Jones, 1981). Similarly, at middle and high 
latitudes wintertime surface temperatures may show a slightly larger 
increase than summertime temperatures, but the variability of wintertime 
temperatures, or noise, is more than twice as large as that in summer* 
This suggests that summertime temperature response may have a larger 
signal-to-noise ratio. It will also be important to distinguish between 
temperatures over land and over the ocean. 

It has been argued that temperature averaged over the lower tropo- 
sphere, rather than surface temperature, may be more representative of 
changes in the radiation balance because of the greenhouse gases. 
Unfortunately, good upper-air coverage does not go back beyond about 
1950, and even now it is deficient over the oceans. So far, satellite 
indirect soundings have not been used to fill in the gaps in the 
climatological record. The situation is worse in the stratosphere 
where the global rocketsonde sounding system has been in operation for 
only a relatively short time and is on the point of being dissolved 
altogether. In view of the difference between tropospheric and strato- 
spheric temperature responses to increased CO 2 , both stratospheric 
and tropospheric temperatures should be monitored. 

Atmospheric fluxes of radiation should also be monitored. The 
monitoring of upward shortwave (reflected and scattered solar) 
radiation and emitted infrared radiatioti at the top of the atmosphere 
deserves special attention. Satellites are potentially powerful tools 
for monitoring these parameters. However, interpretation of most 
satellite measurements to the accuracy required to explain temperature 
changes of a few tenths of a degree appears difficult, as this requires 
that absolute changes in the radiation budget of the order of 0.2% over 
a period of years be detected. Detailed analysis of the spectrum of 
outgoing infrared radiation may, however, provide useful information. 

Clouds may also influence long-term climate variations through their 
radiative effects. To illustrate their potential importance, changes 
in global cloud cover of a few percent could mask the warming effect of 
doubled carbon dioxide. Unfortunately, it is difficult to determine 
with confidence the three-dimensional distribution of clouds because of 
their ill-defined boundaries and complicated configurations. Instead, 
one can monitor the equivalent blackbody temperature of the cloud 
ensembles by measuring their outgoing radiation in one or more 
atmospheric "windows. 11 From this it should be possible to calculate 
not only the upward flux of thermal radiation but also the radiatively 
effective cloud height when the vertical distribution of temperature in 
the atmosphere is given. This, and knowledge of the planetary albedo, 
allows one to evaluate important aspects of the influence of the cloud 
cover on the radiation balance of the Earth-atmosphere system. 

Finally, the precipitable water content of the atmosphere will change 
with C02 increases. Model calculations indicate that the atmospheric 
precipitable water will increase 5-15% as the climate warms in response 
to doubled C(>2 concentrations (Manabe and Stouffer, 1980; Wetherald 
and Manabe, 1981). Observations of this parameter may, therefore, help 



353 

confirm whether the numerical models are properly simulating the role 
of water-vapor processes in contributing to climate change, although 
changes of precipitable water in the near future will, of course, be 
much less than for doubled C0 2 . 

5.3.3.2.1 Global Mean Surface Air Temperature 

Sensitivity. When it is assumed that the CO 2 content of the atmo- 
sphere is doubled and statistical thermal equilibrium is achieved, the 
more realistic of climate modeling efforts predict a global surface 
warming of between 2 and 3.5C, with greater increases at high lati- 
tudes; if one allows for more feedback mechanisms, a range of 1.5 to 
4.5C is suggested (Climate Research Board, 1979; COyciimate Review 
Panel, 1982) . The results from climate models also suggest that the 
C02-induced warming is approximately proportional to the increase of 
the logarithm of the C0 2 concentration in the atmosphere. 

Rate of Change. The large thermal inertia of the oceans will probably 
delay the response of the atmospheric temperature to an increase of the 
C0 2 concentration in the atmosphere. Since the C0 2 -induced warming 
penetrates deeper into the ocean as time passes, the effective thermal 
inertia of the ocean depends on the time scale under consideration. 
Therefore, the length of the delay in the climatic response depends on 
the rate of the C0 2 increase and can change with time. By use of a 
simple model of the atmosphere-ocean system, Hoffert et al. (1980) 
recently estimated the temporal variation of the atmospheric tempera- 
ture in response to a predicted increase of the C0 2 concentration. 
According to their results, the delay of response is approximately 
10-20 years during the period of A.D. 1980-2000. Transient responses 
may also vary zonally and regionally because of the ocean's thermal 
inertia (Schneider and Thompson, 1981; Thompson and Schneider, 1982) . 

Signal-to-Noise Ratio. It is expected that CO 2 - induced change of 
surface air temperature will be positive over most of the Earth's 
surface, whereas the trend of the natural temperature variation changes 
sign from one geographical location to another. Therefore, the 
signal-to-noise ratio of area-averaged surface air temperature should 
increase as the area for the averaging increases. For this reason, the 
global mean surface air temperature is one of the most promising 
quantities for the early detection of the C0 2 climate signal. 

According to the recent study of Jones et al. (1982) , the standard 
deviation of the annually averaged area mean temperature over the 
entire northern hemisphere is about 0.86C, while surface temperature 
anomalies for the northern hemisphere are typically +0.2-0.4C (Clark 
et al., 1982). Depending on assumptions of temperature sensitivity to 
increases in CO 2 concentration and ocean thermal lag, an increase of 
atmospheric CO 2 concentration to between 400 and 450 ppm should raise 
the global average surface air temperature distinctly beyond the 
expected natural fluctuations of surface temperature. 



354 

Adequacy and Availability of Past Data Base* Long, reliable, and 
representative temperature records are needed. Continuing analysis and 
estimates of error of the temperature record are called for; dissemina- 
tion of relevant records is generally good. Analyses of the temporal 
variation of the annually averaged global (or hemispheric) mean surface 
air temperature during the past 100 years have been made by many authors 
by use of instrumental data (see Section 5.2.2.1), with significantly 
differing results, 

Spatial Coverage and Resolution of Additional Measurements Required. 
Estimates of hemispheric (or global) mean surface air temperature are 
subject to large errors because of the spar series s of the observational 
network, particularly over the oceans. Improved observational coverage 
would not only improve the representativeness of hemispheric or global 
averages but would also permit monitoring of regional averages and 
differentiation between changes over ocean and over land. For example, 
model simulation (e.g., Manabe and Stouffer, 1980) indicate larger 
CC^-induced temperature changes in polar regions than in lower lati- 
tudes, and transient responses over the ocean may be expected to differ 
from those over land (Schneider and Thompson, 1981; Ramanathan, 1981) . 
For these reasons, the observational network of surface air temperature 
measurements employed in monitoring and detection studies should be 
supplemented by determinations of sea-surface temperatures from satel- 
lite observations, despite the fact that sea-surface temperature is not 
identical with surface air temperature. 

Frequenc^ of Measurements Required. One or preferably two observations 
per day will suffice. 

Feasibility. Satellite measurements are currently available. However, 
further improvement of the accuracy of sea-surface temperature deter- 
mination is required in order to improve estimates of surface air 
temperature. 

5.3.3.2.2 Tropospheric Temperature Distribution 

Sensitivity. The CC>2-induced change of tropospheric temperature has 
been estimated by use of general circulation models of climate (e.g., 
Manabe and Wetherald, 1975; Manabe and Stouffer, 1980). According to 
these studies, the warming of the surface layer of the atmosphere is 
expected to be particularly large in high latitudes (i.e., 2-3 times 
larger than the increase of global average temperature) . The high- 
latitude warming decreases sharply with increasing altitude. In low 
latitudes, the CO 2 -induced warming is predicted to be somewhat 
smaller than the global average. The warming of the tropical 
troposphere will increase significantly with increasing altitude. 

Rate of Change. Since the troposphere is closely coupled to the 
Earth's surface through dry and moist convective transfer of heat, the 
response time of the tropospheric temperature is not very different 
from that of global surface air temperature discussed in the preceding 



355 

subsection. It is, however, probable that the response time of surface 
air temperature over continents is somewhat shorter than that over 
oceans because of the differences in effective thermal inertia between 
ocean and continent. The response time may also have latitudinal 
variation because the speed of the penetration of thermal anomaly into 
the ocean depends on latitude. 

Signal-to-Noise Ratio. The signal-to-noise ratio for the CC>2-induced 
warming of zonal mean surface air temperature was recently estimated by 
Madden and Ramanathan (1980) and Wigley and Jones (1981) . The results 
from these studies suggest that the signal-to-noise ratio of the CO 2 - 
induced warming of zonal mean surface air temperature is at a maximum 
in middle latitudes in summer when the magnitude of natural temperature 
fluctuation is relatively small. It is desirable to conduct similar 
analyses for the zonal mean temperature of the troposphere. 

Adequacy and Availability of Past Data Base. Tropospheric temperatures 
have been observed by radiosondes on a large scale since the Second 
World War. More recently, satellites have provided a dense coverage of 
remotely sensed soundings, although their accuracy is inadequate to 
determine trends. Angell and Korshover (1978a) estimated the temporal 
variation of the globally averaged temperature in the surface to 
100-mbar layer for the period from 1958 to 1976. They used radiosonde 
data from 63 stations that are fairly evenly distributed. The recent 
compilation of upper-air data by Oort (1982) should also prove valuable 
for the determination of the past trend of tropospheric temperature. 

Spatial Coverage and Resolution of Additional Measurements. The cover- 
age over the oceans of radiosonde observations of atmospheric tempera- 
ture is sparse. To overcome this deficiency, the network of radiosonde 
observation should be augmented with satellite measurements of infrared 
or microwave radiances (to be converted to temperature through appro- 
priate inversion techniques). 

A spatial resolution of 250 km is desirable. 

Frequency of Measurements Required. Two observations per day are 
desirable. 

Feasibility and/or Existence of Technical Systems; Continuity. 
Currently, radiance data from satellites are routinely inverted to 
vertical temperature distributions of the atmosphere, which are used as 
input data for the daily numerical weather forecasts. These data might 
be used for the long-term monitoring of tropospheric temperature; 
careful assessment of the accuracy of current and proposed temperature 
determination from satellites should be made in the light of expected 
C02- induced change of climate. 

5.3.3.2.3 Stratospheric Temperature Distribution 

Sensitivity. Results from both radiative-convective models and general 
circulation models indicate that a cooling of the stratosphere could 



356 

result from an increase of the C0 2 concentration in the atmosphere. 
This cooling contrasts with the warming expected to occur in the tropo- 
sphere. Therefore/ the simultaneous monitoring of temperature not only 
of the troposphere but also of the stratosphere should yield valuable 
information for distinguishing CO 2- induced climate change from climate 
change caused by other factors. 

According to Pels et al. (1980) , cooling due to the doubling of the 
atmospheric CO 2 concentration would be about 7C and 11C at altitudes 
of 30 km and 45 km, respectively. 

Rate of Change. Since the thermal coupling between the stratosphere 
and troposphere is relatively weak, the response time of stratospheric 
temperature is not prolonged by the large thermal inertia of oceans as 
is the case with respect to the tropospheric temperature. According to 
results from time integration of a one-dimensional/ radiative-convective 
model of the atmosphere, the response time (e-folding time) of the 
stratosphere is on the order of a few months (see, for example, Manabe 
and S trickier, 1964) . 

Signal-to-Noise Ratio. The standard deviation of natural temperature 
fluctuation in the stratosphere has a large seasonal variation, being 
small in summer and large in winter. In contrast, the C0 2 - induced 
change of stratospheric temperature estimated by model experiments has 
relatively small dependence on season. Therefore, it is expected that 
the signal-to-noise ratio of the CO2-induced change of stratospheric 
temperature will be at a maximum in summer. In assessment of signal- 
to-noise ratio, it is necessary to know the standard deviations of the 
temperature fluctuation in the middle and upper stratosphere. 

Adequacy and Availability of Past Data Base. Recently, Angell and 
Rorshover (1978b) estimated global temperature variation in the 100-30- 
mbar layer between 1958 and 1977. It is desirable to extend similar 
analyses to the 10-mbar level where the CO y- induced cooling is 
expected to be substantial. In addition, the data from chopper radio- 
meter measurements from satellite and rocketsonde observations of 
temperature in the stratosphere and mesosphere should be analyzed in 
order to determine the recent trend of temperature in the middle 
atmosphere . 

Spatial Coverage and Resolution of Additional Measurements Required. 
It is recommended to monitor the future variation of temperature in the 
middle and upper stratosphere where the magnitude of the C0 2 -induced 
cooling is expected to be large. A spatial resolution of 500 km is 
desirable. This could be accomplished by observation of stratospheric 
temperature from satellite (e.g., by limb measurements), augmented by 
observations by radiosonde and rocketsonde. It is expected that these 
observations will yield a global distribution of stratospheric tempera- 
ture at various altitudes. 

Frequency of Measurements Required. One observation per day is 
adequate . 



357 

it> ility and/or Existence of Technical Systems? Continuity. Current 
S vat i on by radiosonde and satellite should be continued. In addi- 
*on rocketsonde observation of stratospheric temperature should be 
> tinued at least at a few key locations in order to calibrate the 

rature determined by inversion technique from radiance data 
" i ne a from satellites. Careful assessment of the accuracy of the 
s termination of stratospheric temperature from satellites should be 

tele . 

3 3*2.4 Upward Terrestrial Radiation and Reflected Solar Radiation 
: the Top of the Atmosphere 

*nsitivit Infrared emission to space and reflected solar energy 
-om Earth depend primarily on clouds and surface characteristics, 
'idels indicate that total precipitable water will increase, sea- 
ir-Lce temperature will increase, sea ice and snow cover will change, 

f ratosoheric temperature will decrease as C0 2 rises. Climate 

ls cannot reliably tell us, at present, the impact of C0 2 change 
cloud amount, height, and type, although we know that key parameters 

cloud formation, such as the water content of the atmosphere and 

atmospheric lapse rate, will change. 



ate of Change. Since most solar energy is absorbed in the oceans, the 
^nL time is linked to the thermal inertia of the oceans. Regional 

^cfr^SuSr^ ^seTkL 1 cycxe, shouU b. 

tudied as well. 

Natural variability of the emitted and reflected 
.mf Scales LS been'estimated by Stephen, j et al. 

;TLr^rr^^ 

[Q py iat *-" ^ .^^j ..^.s *4--irm llkelV 



tit 



n 



j.^vj_/ . AH *v~*- -- aeometrVf ana cet^cu.** j.w^* 

ave a semiannual wave forced by E ^" l " S ^ d 9 % f lec t e d radiation likely 

ave quasi-random variations in emittea a ^ ^ whole , the annual cycle 

ue to cloud-cover changes. For the gio ih e do it is +1.5%. For 

f emitted IR radiation is +4 W/m^ and for a ^^ albedo varia- 

egions of the Earth the natural noise i a common in northern 

.ions of 15% (absolute units) about a mean ^ ^ currently < 

olar regions, for example. The ^2"J; n decade or two, suggesting 

inknown but is likely to be small in the ne 

i low signal-to-noise ratio. 

Ldeguacy and Availability of Data Base. ^JgJ^th 1 intermittent 
radiation budget measurements begins in tn ^^^ fche Nimbus 6 
lata from various satellite e^ 6 "* 6 ^*^ da ta set to the present, 
md Nimbus 7 experiments provide a on "" overla p with the improved ERBE 
Cf Nimbus 7 lasts two more years, it wi adequate for the 
experiment planned for 1984-1986. These data 



358 

CC>2-related task. However, it is urgent to ensure continued ERBE-type 
measurements beyond 1986. Plans to do so have not yet been finalized. 

Spatial Coverage and Resolution of Additional Measurements Required. 
Satellite coverage is global. A few high-quality surface radiation 
stations, as in the U.S. Geophysical Monitoring for Climatic Change 
(GMCC) network, provide ground checks to complement the satellite 
observations. 

Frequency of Measurements Required. Daily satellite observations are 
desirable at as many local times as possible in order to account for 
diurnal effects (i.e., non-sun-synchronous satellite orbits). 

Feasibility and/or Existence of Technical Systems; Continuity. The 
Nimbus 6 and 7 experiments demonstrated high-precision proof of concept. 
Data are also being provided by the AVHRR instruments on NOAA opera- 
tional satellites. The ERBE will carry improved instrumentation and 
offer needed improvements in local time sampling and in-orbit calibra- 
tion. As noted above, it is important that ERBE-type measurements be 
planned now to continue beyond 1986. 

5.3.3.2.5 Precipitable Water Content of the Atmosphere 

Sensitivity. Model calculations predict that atmospheric precipi table 
water will increase 5-15% as the climate warms in response to doubled 
C0 2 concentrations (Manabe and Stouffer, 1980; We the raid and Manabe, 
1981) . The radiative effects resulting from this increase are a cen- 
tral aspect of the projected climatic response leading to the predicted 
warming due to increasing C(>2 concentrations. Observation of this 
parameter will, therefore, help to confirm whether the numerical models 
are properly simulating the role of water vapor processes in contri- 
buting to climatic change. 

Rate of Change. By the Glaus ius-Clapeyr on equation, the fractional 
change in saturation water vapor mixing ratio (and therefore in 
precipitable water vapor if relative humidity remains constant, which 
is usually assumed) is approximately proportional to the fractional 
change in atmospheric temperature. Since most water vapor is present 
in the troposphere, the response time for the increase in water vapor 
should be about the same as for tropospheric temperature (except for 
the possible influence of year-to-year water storage in the land 
surface layers). If constant relative humidity is assumed, the 
response times would be identical. 

Signal-to-Noise Ratio. The intra-annual range in 5-year averages of 
mean monthly northern hemisphere specific humidity is from 2.17 g/kg in 
February to 3.81 g/kg in August (Oort and Rasmusson, 1971). The inter- 
annual variation in annual average northern hemisphere specific humidity 
is about 0.05 g/kg (Oort, 1982). Climate models project that a doubling 
of C0 2 concentrations would increase the specific humidity by about 
0-3 g/kg (Manabe and Wetherald, 1980), so that the effect should clearly 



359 

Become evident. For a more complete analysis of signal to noise r infor- 
lation on the latitudinal distribution of the expected signal from 
lodels and noise from observations is needed. 

Adequacy and Availability of Past Data Base. The new 15-year global 
lata base of radiosonde-derived atmospheric specific humidity by Oort 
>ffers the potential to develop important baseline data. Development 
)f a global or hemispheric data base extending back before about 1950 
Ls doubtful, however, owing to the limited number of vertical profiles. 
Because the vertical soundings are taken on a fixed 12-hourly (or 
24-hourly) basis only at radiosonde stations, adequate definition of 
the global integral may not be possible; however, the relatively long 
Lifetime of atmospheric water (~10 days) and recent evidence that 
the radiosonde network temperature record is in quite good agreement 
with the more extensive surface network may ameliorate these dif- 
ficulties and permit at least the detection of changes. 

Spatial Coverage and Resolution of Additional Measurements Required. A 
data set based on satellite measurements of atmospheric water vapor 
(whether direct or indirect) would be useful in order to provide 
adequate global coverage and to reduce variations introduced because of 
sampling errors in the present surface network. Prabhakara et al. 
(1982) , for example, have developed a global map of precipitable water 
based on Nimbus 7 microwave measurements. Monthly and latitudinal 
averages should be developed since model results will probably indicate 
that the fractional increases in precipitable water will be a function 
of latitude. 

An alternative approach to direct or indirect measurement of water- 
vapor amount would be to measure the changes in the radiative flux 
expected to result from the projected changes in C0 2 and, primarily, 
water-vapor amount. At the surface, downward infrared radiation is 
projected to increase 15-20 W/m 2 when climate has reached equilibrium 
after a doubling of C02 concentrations; this is about a 5% increase 
in total downward radiation. At the top of the atmosphere, total upward 
infrared radiation will likely change only about 1% under similar 
conditions in response to small planetary albedo changes. To improve 
the signal-to-noise ratio, consideration of expected changes in the 
spectral distribution of the infrared radiation must be undertaken. 
Here the operational AVHRR on current NOAA satellites and other satel- 
lite programs may provide some useful information. Until detailed 
investigations are undertaken, however, the preferred option is to 
monitor atmospheric water vapor amount directly (from radiosondes 
and/or high-resolution spectral measurements on satellites) . 

Frequency of Measurements Required. Measurements once or twice daily 
should be sufficient to develop monthly averages. Special care will 
have to be taken, however, so that account is taken of the presence and 
extent of clouds in the soundings. 

Feasibility and/or Existence of Technical Systems; Continuity. The 
continuation of the radiosonde network is assured for the purposes of 



360 

weather prediction. Surface-based microwave radiometry may provide 
additional data in the future (Skoog et al., 1982). Improved coverage 
over the oceans/ particularly in the southern hemisphere f would be 
helpful* Nimbus 7 and Seasat have demonstrated the feasibility of 
satellite water-vapor measurements over the ocean (Prabhakara et al. / 
1982; Alishouse, 1983) . Research into satellite detection systems is 
currently under way and should be continued. Improvements will likely 
require either detailed spectral measurements by new satellites or 
clever computation of the ratios of broadband fluxes now being measured 
(e.g., Rosenkranz et al., 1982). Compilation of the integral water 
vapor amounts used to initialize daily global weather forecasts should 
be started and compared with integrals based on the radiosonde network. 
Monitoring of precipitable water content will be a challenging task, 
but we believe that the problem should be addressed; an approach com- 
mencing with a feasibility study, evaluation of instrument errors, and 
similar questions is appropriate. 

5.3.3.2.6 Equivalent Emission Temperature (Cloudiness) 

Sensitivity. One of the important factors that can control the 
long-term variation of climate is cloud cover. For example, an 
increase in cloud cover exerts two opposing influences on climate. On 
one hand, it tends to produce cooling of the Earth by reflecting a 
large fraction of insolation. On the other hand, it contributes to the 
warming of the Earth by reducing the outgoing terrestrial radiation at 
the top of the atmosphere. Small percentage changes in global cloudi- 
ness could thus accentuate or counteract a C0 2 - induced warming, 
especially at the regional scale. Unfortunately, it is difficult to 
determine with confidence the needed information on the three- 
dimensional distribution of cloud cover, because cloud cover often has 
ill-defined boundaries and complicated configuration. As an alter- 
native to direct monitoring of cloud cover, one can monitor the 
equivalent temperature for the upward window radiation. From equiva- 
lent emission temperature it is possible to compute both the upward 
flux of blackbody radiation from a cloud top and the effective cloud 
height when the vertical distribution of temperature in the atmosphere 
is given. By monitoring the long-term variation of both effective 
cloud height and planetary albedo, one can evaluate the net effect of 
the changes in cloud cover and optical properties of the Earth's 
surface. 

Rate of Change. Changes should occur in close association with C02 
and climate changes. 

Signal-to-Noise Ratio, Since the temperature change for doubled C0 2 
is expected to be about 3C, one might estimate that global mean 
equivalent emission temperature should be measured to an accuracy of a 
few tenths of a degree. It has not been possible, however, to obtain a 
reliable estimate of the signal-to-noise ratio for the CO 2 -induced 
change of the equivalent emission temperature (or effective height of 
the source of upward window radiation) for the following reasons: (a) 



361 

In view of poor performance of current general circulation models of 
the atmosphere in simulating the global distribution of cloud cover and 
its seasonal variation, it is premature to trust the CO y- induced 
changes of cloud cover and effective emission temperature as determined 
by such models, (b) The analysis of the natural variability of effec- 
tive emission temperature for upward window radiation is not available. 

Availability of Past Data Base. Over a decade of full-disk geosta- 
tionary satellite visible images has been archived at the University of 
Wisconsin. Window radiance data from NOAA operational satellites are 
routinely archived by the National Environmental Satellite Service r 
which regularly publishes analyses including both images and tabulated 
data. 

Spatial and Temporal Resolution of Additional Measurements Required. 
The spatial and temporal resolution of future satellite observation 
should be determined in the light of careful assessment of sampling 
error involved in the time (or space) averaging of window radiance as 
obtained from a satellite. The past observation of this variable by a 
geosynchronous satellite provides ideal data for such an assessment. 

Frequency of Observations Required. The frequency of observations will 
be determined by the sampling error studies described above. 

Feasibility and/or Existence of Technical Systems; Continuity. This is 
an area for further study. Plans and feasibility studies should be 
made in conjunction with the recently initiated International Satellite 
Cloud Climatology Project (ISCCP) (World Meteorological Organization, 
1982b) . 



5.3.3.3 Cryospheric Parameters 

The features of the cryosphere include snow, sea ice, glaciers, ice 
sheets, permafrost, and river and lake ice. Perennial ice at present 
covers about 7% of the world oceans and 11% of the land surface, almost 
entirely in the polar regions. Seasonal snow and ice, however, occupy 
15% of the Earth's surface in January, at a time when Antarctic sea ice 
is near its minimum extent, and 9% in July when there is almost no snow 
cover in the northern hemisphere* Table 5.10 shows the global distribu- 
tion of ice and snow. The volume of water locked up in the Antarctic 
and Greenland ice sheets could potentially raise the world's mean sea 
level by 77 m. 

The general problems of possible snow and ice responses to a carbon 
dioxide- induced warming were reviewed by Barry (1978, 1982) and Kukla 
(1982) in terms of the sensitivity of individual components of the 
cryosphere. The response of snow and ice covers to climatic change 
varies greatly in terms of time scale. Typical residence times of 
solid precipitation in the various reservoirs are approximately 10""^ 
to 1 year for seasonal snow cover, 1-10 years for sea ice, and 
10 3 -10 5 years for ground ice and ice sheets. In each case certain 



362 
TABLE 5.10 Distribution of Ice and SnowS. 



Volume Sea-Level 


Area (106 km 3 Equivalent 
(106 km 2 ) water) (m) 


Land Ice 




Antarctica, 


12.2 25 70 


Greenland 


1.8 2.7 7 


Small ice caps and mountain glaciers 


0.5 0.12 0.3 


Ground Ice (excluding Antarctica) 




Continuous 


7.6 


Discontinuous 


17.3 1 


Sea Ice 




Arctic: max. 


15 


min. 


5 


Antarctic: max. 


20 


min. 


2.5 


Total Land Ice, Sea Ice f and Snow 




Jan: N. hemisphere 


58 


S. hemisphere 


18 


July: N. hemisphere 


14 


S. hemisphere 


25 


Global mean annual 


59 



^SOURCE: Hollin and Barry (1979) . 

^Excludes peripheral, floating ice shelves (which do not affect sea 

level) . 

Roughly 10% of the Antarctic ice is in West Antarctica and 90% is in 

East Antarctica. 



phases of the seasonal regime are particularly critical for the 
occurrence of snow and ice and their response to climatic variations. 
Of primary importance are the times of seasonal temperature transition 
across the 0C threshold (-1.8C in the case of seawater) . Other 
threshold effects that influence radiative and turbulent energy 
exchanges arise as a result of the large albedo differences between 
snow cover (about 0.80) and snow-free ground (0.10-0.25) or between ice 
(0.65) and water (0.05-0.10). 

Climate research indicates that doubling the C0 2 concentrations 
will lead to a significant reduction in the extent of snow cover and 
sea ice with perhaps, if the warming persists, melting and deteriora- 
tion of the major polar ice sheets. Sea- ice and snow-cover extent can 
be monitored routinely from satellites, and it has been shown that the 
mass balance of the large continental ice sheets of Antarctica and 
Greenland could also be monitored from satellites. These three cryo- 
spheric parameters are the most promising ones to monitor. Others, 
like smaller glaciers and river and lake ice, for example the dates of 



363 

freezeup of the latter, are in most cases probably too noisy to be used 
as good indicators of climate change. Several circulation models 
suggest that increased C02 concentrations will lead to winter warming 
in the polar regions that is several times as large as in middle and 
low latitudes, Cryospheric conditions , particularly those responding 
rapidly to climatic change ? may thus be excellent early indicators of 
C02-induced effects. It is, nevertheless, important also to mention 
other, slower-responding phenomena, because of the environmental 
significance of potential changes that may be detected in them. 

5.3.3.3.1 Sea-Ice Cover 

Sensitivity. Model calculations by Manabe and Stouffer (1980) show 
sea-ice cover, both in areal extent and thickness, to be a sensitive 
indicator of climate change. They assume an increase of four times the 
present C02 concentration and therefore project a very large warming 
in the polar region. This results in the sea- ice cover of the Arctic 
Ocean being reduced to a seasonal ice cover, which reforms in winter. 
Budd (1975) calculates from empirical data in Antarctica that an annual 
change of 1C in mean temperature corresponds to a 70-day variation in 
the duration of sea ice at the margin and a 2.5 latitude variation in 
maximum extent. Observations (Vinnikov et al., 1981) and paleoclimatic 
reconstructions using sediment data (Hays, 1978) confirm the relation- 
ship between temperature and sea- ice extent. Despite the thicker ice 
in the Arctic (~3 m) compared with that in the Antarctic (~2 m) , 
empirical and modeling results seem to indicate that the ice extent 
responds rapidly (on a seasonal time scale) to climate changes. 

Rate of Change. The natural variability of ice extent is large, and 
Zwally et al. (1983b) have not detected a systematic decrease of ice 
extent in Antarctica since 1973. Data for the Arctic also so far do 
not indicate any clearcut effects due to a CO 2 warming there in the 
past 25 years. 

Signal-to-Noise Ratio. Sea-ice extent is a noisy parameter when con- 
sidered over short time scales or small space scales, since it is 
determined by numerous environmental parameters and by both dynamic and 
thermodynamic processes (Pritchard, 1980). In the Antarctic, where sea 
ice displays a wide seasonal variation in extent, recent studies 
(Zwally et al., 1983b) have shown no systematic trend. Decreases 
between 1973 and 1980 were within 1 standard deviation of the long-term 
mean (Budd, 1980) and have been followed by increases since 1980. In 
the largely enclosed Arctic Ocean, variations in the ice extent are 
more limited on a seasonal basis, and ice thickness changes may be the 
first indicator of a climate change. 

Adequacy and Availability of Past Data Base. Accurate data on the 
extent of sea ice in both hemispheres are limited to the satellite 
records of the last two decades or so, although historical data in 
isolated instances, e.g., Iceland (Vilmundarson, 1972) and northern 
Europe (Vinnikov et al., 1981) date back several centuries. Sediment 



364 

core data (Hays f 1978) have extended this data base by many thousands 
of years. The satellite records are too short at present to determine 
definite trends/ but continued monitoring over the next 10-15 years 
should establish whether incipient or proposed trends are significant. 
Data on sea-ice extent are also not yet archived routinely in digital 
form. 
/ 

Spatial Coverage and Resolution of Additional Measurements Required. 
Satellite measurements allow routine integration of the areal extent of 
sea ice in both hemispheres. It would be useful also to have sea-ice 
thickness distributions in both polar regions/ but these measurements 
are at present not feasible for satellites. 

Frequency of Measurements Required. Weekly averages for each hem- 
isphere/ as determined at present/ are adequate/ from which monthly and 
annual means as well as maxima and minima can be derived. 

Feasibility and/or Existence of Technical Systems; Continuity. Tech- 
nical systems exist to carry out routine sea-ice monitoring from 
spacecraft. All-weather and night capability is essential/ since both 
polar regions are dark for prolonged periods/ and seasonal cloud 
systems are extensive. Microwave or radar systems are needed/ but 
there is likely to be a hiatus in the launch of U.S. spacecraft with 

* such systems that can perhaps be filled only by using European or 

Japanese satellites. 

5.3.3.3.2 Snow Cover 

Sensitivity. For snow cover/ the C0 2 signal is more difficult to 
interpret than for sea ice/ since the effects of C0 2 -induced warming 
on snowfall and snow cover will vary with latitude. In low and middle 
latitudes/ where the occurrence of snow rather than rain is frequently 
marginal/ warming will decrease the frequency of the snowfall and the 
duration of snow cover on the ground. In high latitudes/ snowfall is 
limited by the frequency of cyclonic incursions and the moisture content 
of the air/ and there is a tendency for warm winters to be snowy/ as 
for example at Barrow/ Alaska (Barry/ 1982) . The year-to-year vari- 
ability of snow cover in the northern hemisphere is large/ but global 
warming could eliminate the occurrence of snow completely in broad 
areas of low snowfall frequency (Dickson and Posey/ 1967) / increase it 
at higher latitudes/ but also possibly result in an overall increase in 
the length of the snow-free season in the higher latitudes due to 
warmer summer temperatures. 

Rate of Change. The generally thin snow cover of the Arctic requires 
little energy for melting and can therefore respond rapidly to changes 
in the energy balance triggered by C0 2 -induced warming. The duration 
of snow cover at high latitudes is determined primarily by summer tem- 
peratures/ since the depth of snow is not highly variable from year to 
year (Barry/ 1982) . Typically/ a 30-40-cm snow cover in the Arctic 
disappears in about 10 days from the start of melting and requires 
about 2-3 kJ cm" 2 (Weller and Holmgren/ 1974) . 



365 

Signal-to-Noise Ratio. The signal-to-noise ratio of snow cover extent 
in the Arctic is likely to be high, because snowfall at high latitudes 
is highly variable in space and time. For example, one day's precipita- 
tion amount may be a large percentage of the total precipitation in 
some areas (Maxwell, 1980). As discussed above, snow cover may either 
increase or decrease, depending on latitude, geographical location, and 
change in circulation patterns caused by CC>2 effects. 

Adequacy and Availability of Past Data Base. Information on the dura- 
tion of snow cover and the last date of snow on the ground is available 
for Canada (Potter, 1965) and in maps of probability for the northern 
hemisphere (Dickson and Posey, 1967) . Kukla (1981) and Matson and 
Wiesnet (1981) have recently compiled satellite information on monthly 
snow limits. Snow depth data are not available in convenient archives, 
although they are recorded in written synoptic weather reports, and 
selected mapping has been performed by the British Meteorological 
Office since 1962 for Eurasia and since 1971 for North America (Taylor, 
1980) . 

Spatial Coverage and Resolution of Additional Measurements Required. A 
regular program of mapping global snow cover extent and depth with a 
higher time and space resolution is needed for present purposes. A 
50-km grid is required. Frequent satellite observations with a 
horizontal resolution of 1-4 km would make this possible for snow-cover 
extent. Snow-depth data from synoptic weather reports currently have a 
coarse resolution, and refinement is desirable here as well. 

Frequency of Measurements Required. Snow maps, which at present are 
compiled weekly, are adequate for long-term monitoring studies, although 
studies of synoptic-scale interactions would require daily maps. 

Feasibility and/or Existence of Technical Systems; Continuity. Satel- 
lite systems like those currently in use are adequate for purposes of 
measuring snow cover extent, but at present snow depth cannot be 
measured from space. 

5.3.3.3.3 Ice-Cap Mass Balance Changes 

Sensitivity. The effects of a warming on the Greenland and Antarctic 
ice sheets are likely to be complex (Bentley, 1983? Revelle, this 
volume, Chapter 8) . In the short-run CO 2 - induced climate changes 
could result in either positive or negative transient mass balance 
changes of the ice sheets, depending on regional shifts in temperature 
and precipitation. The potential of C(>2-induced changes in the next 
few decades to initiate disintegration of the West Antarctic ice sheet 
is very small. On the other hand, Ambach (1980) states that a tempera- 
ture increase of only 1.5C will cause a decisive negative change in 
the mass balance of Greenland. Such mass balance changes will in turn 
slowly affect sea level. For a 3C CC^-induced warming, Revelle 
(this volume, Chapter 8) calculates a 60-70-cm rise in global sea 
level, about half due to ablation of the Greenland and Antarctic ice 



366 

caps and about half due to increase in the specific volume of seawater 
resulting from an increase in temperature. We thus need to measure the 
mass balance of these ice sheets in order to understand long-term 
sea-level changes. 

Rate of Change. Because of the large mass and long residence time 
(about 10^-10^ years) of the ice in the Greenland and Antarctic ice 
sheets, their responses to C02 - induced warming will be slow. Based 
on our present knowledge, it appears that a CO 2 -induced warming on 
the century time scale will have only minor consequences for ice sheets, 
but changes in their thickness may be detectable at intervals of 5-10 
years. Detectable shorter-term changes could include higher melting on 
ice shelves, changes in iceberg calving rates, or changes in surface 
gradient on ice shelves near ice rises. Ice shelves have shorter 
response times and provide a first indication of the state of health of 
the ice sheet. 

Signal-to-Noise Ratio. The signal-to-noise ratio is probably quite 
high, since the interiors of the large ice sheets are stable and 
relatively inactive. Noise may be increased by the limits of present 
measurement techniques. 

Adequacy and Availability of Past Data Base. The mass balance of the 
Greenland ice sheet is well known (Ambach, 1980) , and satellite 
measurements, using airborne radio echo sounding (Robin et al., 1977) 
and radar altimetry have begun to give us similar data for Antarctica. 
Previous estimates of the mass balance changes of Antarctica are 
unreliable, varying from positive to negative values. Few data are 
available on the extent of melt features on ice shelves, on calving 
rates of iceberg, or on the response of ice shelves to climatic changes. 

Spatial Coverage and Resolution of Additional Measurements Required. 
Satelliteborne radar altimeter flights in a polar orbit are required 
for at least every 10 of longitude. The vertical resolution of height 
of the ice sheet should be +5 cm. Areas in which to look for changes 
are not only the elevation of the interior of ice sheets but also 
gradient changes near ice shelves and ice rises. 

Frequency of Measurements Required. Changes in the ice-sheet mass 
balance of Greenland and Antarctica should be monitored at 5-year 
intervals. Seasonal features, such as the extent of melt features on 
ice shelves, should be surveyed annually. 

Feasibility and/or Existence of Technical Systems; Continuity. The 
feasibility of making such measurements has been demonstrated by using 
the radar altimeter on the GEOS-3 satellite launched in 1975 (Brooks et 
al., 1978) and by studies based on Seasat data (Zwally et al., 1983a) 
and simulations of possible future satellite systems (Zwally et al., 
1981) . No U.S. satellite currently carries either radar or laser 
altimeters, however, but plans for future satellites should include 
them. 



367 
5.3.3.4 Oceanic Parameters 

The oceans are key elements in the Earth's climate system. However, 
there are still major uncertainties in our knowledge of how the coupled 
ocean-atmosphere system works and r therefore r how it may change when 
C0 2 is added to the atmosphere. The available , though sketchy , 
evidence points toward the fact that the ocean is most probably delay- 
ing the temperature signal of increasing CO 2 by mixing heat downward. 
It is clear from model studies, which up to now have treated the oceans 
quite simply (e.g., Gates et al., 1981; Manabe and Stouffer, 1980), 
that the response of the atmosphere is paced by that of the ocean. 

Thompson and Schneider (1979) and Schneider and Thompson (1981) 
discussed the question of the transient response of the atmosphere to 
C02 using models based on the thermal inertia of the upper layers of 
the oceans in combination with their interaction with deeper waters. 
Similar conclusions about the delay of a warming, i.e., as long as a 
few decades, were reached in reports of the National Academy of Sciences 
(Climate Research Board, 1979; C0 2 /Climate Review Panel, 1982). 
Ramanathan (1981) emphasizes the inadequacy of present coupled models 
for examining the transient response. Work of Fine et al. (1981) on 
tritium penetration into the ocean suggested that the delay time may 
have been underestimated. Regional climate changes will also be 
strongly associated with climatic variation in the ocean (Schneider and 
Thompson, 1981; Bryan et al., 1982; Thompson and Schneider, 1982). 

In order to identify changes in the ocean due to CO 2 warming, a 
long-term measurement program is required. Brewer (this volume, 
Chapter 3, Section 3.2) examines the changes in ocean chemistry to be 
expected from increasing C0 2 . Baker and Barnett (1982) describe the 
physical oceanographic variables that would be expected to respond. 

Of the parameters identified by these authors, which include sea 
level, sea temperatures, salinity, and ocean circulation patterns, the 
first two seem most appropriate for monitoring the possible effects of 
C0 2 -induced warming. Changes in sea level, though not driven by 
thermal expansion alone, may be the best indicator of the global change 
in ocean temperature, because an observational network exists, at least 
in the northern hemisphere, and sea-level data are representative of 
integrated, rather than point, measurements. 

5.3.3.4.1 Sea Level 

Sensitivity. Sea level should be a sensitive indicator of C0 2 
effects. A change of 0.1% of the global land ice cover will result in 
a sea-level change of about 5 cm (Flint, 1971), and increases in ocean 
temperature will presumably accompany increases in atmospheric tem- 
perature; a change of 0.5C in the upper 200 m would increase sea level 
by roughly 2 cm (Baker and Barnett, 1982) . Given a 3C atmospheric 
warming, Revelle (this volume, Chapter 8) estimates a rise in sea level 
about 100 years from now of at least 30 cm, resulting from ocean warm- 
ing, and a probable rise of between 60 and 70 cm, if ice ablation is 
included. Both of these effects will be global in nature, and it may 
be far easier to detect a signal that is coherent in all the oceans 



368 

than to identify one that is rather regional in character. Measurement 
problems exist, however, which are discussed below. 

Rate of Change. Rises in sea level in response to projected C0 2 - 
induced warming will be slow, but much more rapid than recent historic 
rates. Estimates of the rate of sea-level rise so far during this 
century range from 1 to 3 mm/year (see Revelle, this volume. Chapter 
8). Problems of tectonic movement and poor station distribution in 
terms of location, offshore current, and wind systems, for example, 
leave one uncertain as to the reality and meaning of these numbers. 

Signal- to-Noise Ratio. The relatively long time series of sea level 
provide opportunities to estimate signal-to-noise ratios and hence make 
detection of global changes more feasible (cf., Madden and Ramanathan, 
1980). However, there are relatively few sea-level stations in the 
southern hemisphere that possess a long record, and, further, sea-level 
data in huge ocean regions must be reconstructed from limited hydro- 
graphic data These deficiencies will make detection of a truly global 
signal somewhat more difficult. 

Adequacy and Availability of Past Data Base. Long time series of 
sea-level data are available (e.g., Emery, 1980; Revelle, this volume, 
Chapter 8) , but their interpretation is complicated by the problems 
listed above. Nevertheless, these studies suggest a coherent rising of 
sea level on scales of oceanic dimensions. The statistical significance 
of the changes and their relationship to CC>2-induced warming are hard 
to estimate. 

Spatial Coverage and Resolution of Additional Measurements Required. 
It appears feasible to achieve spatial coverage sufficient to ameliorate 
some of the difficulties mentioned above, although problems in the 
interpretation of individual records will remain. Measurements are 
needed at all open ocean island locations; primarily lacking at present 
are islands in the Atlantic and Indian Oceans. Also needed are stations 
around the Antarctic continent. Global sea-level coverage will be 
available from the various altimetric satellites now being proposed for 
the late 1980s. The estimated accuracy of these, +10 cm, will be too 
low for useful estimation of C0 2 effects in the next one or two 
decades; however, over the long term (i.e., a century) such satellite 
measurements will be helpful. 

Frequency of Measurements Required. Since most sea-level measurements 
are made for tidal prediction, the frequency of measurement is more 
than adequate for long-term sea-level change. It is critical to keep 
the measurement going for a long time (decades) . 

Feasibility and/or Existence of Technical Systems; Continuity. Tide- 
gauge measurements are simple and have been carried out for a long 
time. The existing technology that permits unattended operation for 
months to years with data recording on tape cassettes is entirely 
adequate for the purpose. 



369 
5.3.3.4.2 Sea Temperature 

Sensitivity. Most simulations of increased atmospheric CO 2 show a 
substantial warming of surface air temperature over the globe. The 
effect is most pronounced in high latitudes. As with sea level r change 
in sea-surface temperature resulting from this air temperature increase 
should be global in nature. However, its magnitude will have a strong 
regional character (Baker and Barnett, 1982) , owing to regional varia- 
tion in vertical mixing and diffusion and the relative importance of 
different physical processes in the ocean heat budget; detection of a 
C0 2 -induced temperature signal in the oceans will thus be difficult. 
Certainly, the nature and magnitude of such a signal merits further 
study . 

Rate of Change. Response time of sea temperatures to CC>2 effects 
could be relatively slow owing to the thermal inertia of the oceans. 
Data on sea-surface temperature (SST) for the Indian, North Atlantic, 
and North and Tropical Pacific Oceans show perhaps a 1C rise over the 
last 80 years in all oceans (Baker and Barnett, 1982) . The change may 
be smaller because of possible errors due to the gradual conversion 
from measurements by bucket thermometer to ship's injection thermom- 
eters; the latter consistently showing SSTs warmer by 0.4C. 

Signal-to-Noise Ratio. Current data archives on the surface tempera- 
ture field of the world oceans are quite extensive and should make 
proper determinations of signal-to-noise ratios relatively simple. The 
historical record of surface temperature and subsurface temperature 
needs to be analyzed to determine the type of signal that could be 
detected over the background. Estimates of regional signal-to-noise 
ratios also need to be made; these have not been performed so far. The 
effects of mesoscale eddies add a large noise to subsurface ocean 
temperature that is generally not so prominent in the SST field. 

Adequacy and Availability of Past Data Base. Measurements of sea- 
surface temperature have been made in many regions of the world for the 
past 100 years from ships, islands, and coastal stations, although many 
of these data are crude and unrepresentative. More complete analyses 
of this record should be undertaken. Analyzed data covering the North 
Atlantic and Pacific are available back to about 1948 (Walsh and Sater, 
1981; Bunker, 1980). Subsurface temperature fields are much less well 
known. The historical record of subsurface ocean temperature is unfor- 
tunately relatively short, and the data are subject to errors intro- 
duced by changes in the instrumentation (bathythermographs) , which may 
introduce bias into the data sets. 

Spatial Coverage and Resolution of Additional Measurements Required. 
It is now possible to collect data on sea-surface temperatures for all 
oceanic regions of the world. Infrared satellite sensors have been 
used since the early 1970s, and improved satellite analyses are now 
produced daily for all oceanic regions. Programs of direct measurement 
must, however, be maintained for their better accuracy, and new methods 



370 

(e.g., acoustic tomography) should be explored for measuring integrated 
ocean temperature. 

Frequency of Measurements Required, Daily analyses are available from 
satellite infrared sensors; their frequency is adequate. 

Feasibility and/or Existence of Technical Systems? Continuity. It is 
necessary to evaluate rigorously how well the sea-surface temperature 
signal can be extracted from the satellite microwave radiometer. The 
demonstrated accuracy of the spaceborne sea-surface temperature systems 
still remains inadequate to resolve the signals as observed by direct 
measurement. For studies of warming, accuracies of from 0.1 to 0.5C 
are needed, but this accuracy has not yet been achieved. Much of this 
uncertainty comes from surface foam, water vapor, and liquid water in 
the path of the sensor; the new multichannel sensors may show marked 
improvement. Acoustic tomography (Munk and Wunsch, 1982) also offers 
promise of providing integrated ocean temperatures. 



5.3.4 Conclusions and Recommendations 



5.3.4.1 Priority of Parameters to be Monitored 

To help determine the current and projected effects of increasing 
atmospheric C0 2 on climate, we recommend further elaboration and, if 
sound, the development of a monitoring strategy in which many measures 
of the state of the climate systems are monitored and analyzed as an 
ensemble . 

If recent climatic trends are sustained, it seems likely that there 
will be an increasing number of claims to have distinguished a sig- 
nificant warming. The problem will then become increasingly one of 
attribution of cause and effect. The most promising means to achieve 
convincing attribution will be development of reliable records of 
several parameters in addition to temperature. Clearly, for technical 
or cost reasons adequate monitoring of some parameters will be much 
more readily achievable than others. To accomplish early attribution, 
initial emphasis should be given to these. 

While initial emphasis may be placed on parameters that may be 
monitored immediately, cheaply, or easily, it is important over the 
long run to build up a rather complete data base, not only for reasons 
of detection but also for research and for calibration of models of the 
climate system. It may take until 2010 or 2020 to begin to have useful 
data bases on some of the parameters mentioned here, but they should 
not be neglected. Instead, we should anticipate that a monitoring 
program will gradually evolve into a program to verify and calibrate 
crucial aspects of model calculations, especially the numerous projected 
effects of increasing atmospheric CO 2 , for example, sea-level rise 
and changes in rainfall in midlatitudes . 

Based on this initial survey, we summarize our recommendations for 
monitoring in Table 5.11. 



371 

TABLE 5.11 Priority in Monitoring Variables for Early Detection of 
C0 2 Effects 



Monitor ing Monitor ing 

Causal Factors by Climatic Effects by 
Priority Measuring Changes in Measuring Changes in 



First C0 2 concentrations Troposphere/surface 

Volcanic aerosols temperatures (including 
Solar radiance sea temperatures) 

Stratospheric temperatures 
Radiation fluxes at the top 

of the atmosphere 
Precipitable water content 

(and clouds) 
Second "Greenhouse" gases Snow and sea- ice covers 

other than C0 2 Polar ice-sheet mass balance 
Stratospheric and Sea level 
tropospheric ozone 



5*3.4*2 Measurement Networks 

The key to a successful monitoring strategy is a global observation 
system. Satellites are a major component of such a system f and it is 
essential to be able to continue monitoring without interruption on a 
long-term basis the radiative f luxes , the planetary albedo , snow and 
ice extent, and sea-surface temperatures and to improve the spaceborne 
measurements of tropospheric and stratospheric temperatures/ precip- 
i table water content of the atmosphere , mass balance of the polar ice 
sheets and sea level , as well as aerosols, ozone, and other atmospheric 
constituents. 

Many of the satellite measurements that are being made at present 
are difficult to calibrate. Of particular concern are the vertical 
soundings made from spacecraft and the lack of supporting surface-based 
or surface-launched profiling systems. A concern, for example, is the 
dismantling of the global rocketsonde system of stratospheric tempera- 
ture soundings. Some key stations should be retained to calibrate the 
instruments flown on satellites. Similarly, the only reliable method 
of characterizing stratospheric aerosols at present is by the deploy- 
ment of lidar systems. By adding one or two such systems in the 
southern hemisphere to the existing network, and through occasional 
aircraft flights to calibrate the satellite soundings, an adequate 
amount of data could be collected. Other parameters, for example ozone, 
are also inadequately measured at present. Total ozone values derived 
from Dobson spectrometer and satellite profile measurements are not 
enough in themselves, and the existing ozonesonde network must be 
maintained and augmented in data-sparse regions. 



372 

The southern hemisphere presents a particular problem in monitoring 
climatic changes that can only be solved by improving and perfecting 
the present satellite-based sounding techniques. 

Table 5.1 summarizes requirements and technical systems for monitor- 
ing high-priority variables. 

5.3.4.3 Modeling and Statistical Techniques 

The internal physical consistency and relative ease of diagnosis of 
simulated climatic data make the construction of realistic and compre- 
hensive models a prerequisite for the development of a successful 
fingerprinting strategy for the detection of C02-induced climatic 
change. In addition, climate models are needed in order to determine 
the accuracy that is required in monitored climatic variables. Unfor- 
tunately, the C02-induced climatic changes calculated from the various 
current climate models continue to show substantial differences. In 
order to develop an effective monitoring strategy, it is essential that 
further intensive efforts be made to improve climate models by validat- 
ing them against the observed structure and behavior of the ocean- 
atmosphere system and to make effective use of model improvements. 
Another important element is the development of methods for the 
statistical identification of a CO 2 -induced climate signal against 
the background of natural climatic fluctuations. Statistical tech- 
niques applied so far focus on the significance of the signal-to-noise 
ratio, assuming that the data at individual points may be modeled as a 
first-order autoregressive process. Further use should be made of 
significance tests that consider longer-term dependence in the climatic 
time series and that provide estimates of confidence limits. An essen- 
tial ingredient of a successful detection strategy will be the develop- 
ment of techniques that take into account not only the temporal corre- 
lation but also the spatial correlation that is characteristic of 
nearly all climatic variables and that lead to more careful and sophis- 
ticated statistical tests of a possible CC>2 climatic signal. Here 
again, model-simulated data can be used effectively to begin to develop 
and test the statistical procedures that will ultimately have to be 
applied to monitored observational data. However, purely statistical 
inferences will have to be buttressed to the greatest extent possible 
by physical reasoning. 



5.3.4.4 Objective Evaluation of Evidence 

The differing interpretations of the effects of likely C0 2 -induced 
climate changes, as arrived at by different authors (see Section 5.2) , 
often using identical data sets, underline the need for objective 
evaluation of evidence. It is wise to anticipate the need at national 
and international levels for periodic efforts to evaluate evidence and 
arbitrate between divergent opinions, where necessary. Some centers 
that can perform such functions are already in existence as part of 
national and international programs. At these centers climatic indices 



373 

should be collected and compared , and evaluation and improvement of 
techniques for identification of climatic changes should be encouraged. 

REFERENCES 

Alishouse, J. C. (1983) . Total precipitable water and rainfall 

determinations from SEASAT scanning multichannel microwave 

radiometer. J. Geophys. Res. 88; 1929-1935. 
Ambach, W. (1980) . Anstieg der CO 2 -Konzentration in der AtmosphSre 

und Klimaanderuug : MSgliche Auswirkungen auf dem GrSnlSndischen 

Eisschild. Wetter und Leben 32; 135-42. 
Angell, J. K. f and J. Korshover (1978a) . Global temperature variation; 

surface 100 mb; an update into 1977. Mon. Wea. Rev. 106; 755-770. 
Angell, J. K. f and J. Korshover (1978b) . Estimate of global 

temperature variations in the 100-30 mb layer between 1958 and 

1977. Mon. Wea. Rev. 106; 1422-143 2. 
Augustsson, T. r and V. Ramanathan (1977). A radiative-convective model 

study of the O^-climate problem. J. Atmos. Sci. 34;448-451 
Baker, Jr., D. J., and T. P. Barnett (1982). Possibilities of 

detecting CO2-induced effects: ocean physics. In U.S. Department 

of Energy (1982), pp. 301-342. 
Baldwin, B., J. B. Pollack, A. Summers, O. B. Toon, C. Sagan, and W. 

Van Camp, (1976) . Stratospheric aerosols and climatic change. 

Nature 263; 551-555 . 
Barry, R. G. (1978) . Cryospheric responses to a global temperature 

increase. In Carbon Dioxide , Climate and Society, J. Williams, ed. 

Pergamon, Oxford, pp. 169-180. 
Barry, R. G. (1982) . Snow and ice indicators of possible climatic 

effects of increasing atmospheric carbon dioxide* In U.S. Department 

of Energy (1982), pp. 207-236. 
Bates, J. R. (1977) . Dynamics of stationary ultra-long waves in middle 

latitudes . Q. J. Roy. Meteorol. Soc. 103; 397-430 . 
Bentley, C. R., ed. (in press). CO 2 -induced Climate Change and the 

Dynamics of Antarctic Ice. Proceedings of AAAS Symposium, Toronto, 

January 1981, American Association for the Advancement of Science, 

Washington, D.C.. 
Bloomfield, p., M. L. Thompson, G. S. Watson, and S. Zeger (1981). 

Frequency Domain Estimation of Trends in Stratospheric Ozone* 

Technical Report 182. Dept. of Statistics, Princeton U., Princeton, 

N.J. 

Borzenkova, I. I., K. Ya. Vinnikov, L. P. Spirina, and D. I. 

Stekhnovskii (1976) . Variation of northern hemisphere air 

temperature from 1881 to 1975. Meteorol. Gidrol. 7; 27-35. 
Broecker, W. S. (1975). Climatic change; are we on the brink of a 

pronounced global warming? Science 188;460-463. 
Brooks, R. L., W. J. Cambell, R. O. Ramseier, H. R. Stanley, and H. J. 

Zwally (1978) . Ice sheet topography by satellite altimetry. Nature 

274;539-43. 



374 

Bryan, K. , F. G. Komro, S. Manabe, and M. J. Spelman (1982). Transient 

climate response to increasing atmospheric carbon dioxide. Science 

215:56-58. 
Bryson, R. A. (1980). C0 2 , Aerosols, and Modeling Climate of the 

Recent Past. In U.S. Department of Energy (1980), pp. 139-143. 
Bryson, R. A., and G. J. Dittberner (1976). A non-equilibrium model of 

hemispheric mean surface temperature. J Atmos. Sci. 33; 2094-2106. 
Bryson, R. A., and B. M. Goodman (1980). Volcanic activity and 

climatic changes. Science 207; 1041-1044. 
Bryson, R. A., and W. M. Wendland (1970). Climatic Effects of 

Atmospheric Pollution. Global Effects of Environmental Pollution, 

S. F. Singer, ed. Springer-Verlag, New York, pp. 130-138. 
Budd, W. F. (1975) . Antarctic sea ice variations from satellite 

sensing in relation to climate. J. Glaciol. 73; 417-427. 
Budd, W. F. (1980) . The importance of the antarctic region for studies 

of the atmospheric carbon dioxide concentration. In Carbon Dioxide 

and Climate; Australian Research, G. I. Pearman, ed. Australian 

Academy of Science, Canberra, pp. 115-28. 
Budyko, M. I. (1969). The effect of solar radiation variations on the 

climate of the earth. Tellus 21; 611-619. 
Budyko, M. I. (1974). Climatic Changes* Hydrometeoizdat, Leningrad 

(47 pp.); transl., American Geophysical Union, 1977. 
Budyko, M. I., and K. Ya. Vinnikov (1973). Modern climate variations. 

Meteorol. Gidrol. 9; 3-13 . 
Bunker, A. F. (1980). Trends of variables and energy fluxes over the 

Atlantic Ocean from 1948 to 1972. Mon. Wea. Rev. 108; 720-732. 
Callendar, G. S. (1938). The artificial production of carbon dioxide 

and its influence on temperatures. Q. J. Roy. Meteor ol. Soc* 

64; 223-240. 
Callendar, G. S. (1961). Temperature fluctuations and trends over the 

earth. Q. J. Roy. Meteorol. Soc. 87; 1-12. 
Charney, J. G. (1975) . Dynamics of deserts and drought in the Sahel. 

Q. J. Meteorol. Soc. 101;193-202. 
Clark, W. C., ed. (1982). Carbon Dioxide Review; 1982. Oxford U. 

Press, New York, 469 pp. 
Clark, W. C., K. H. Cook, G. Marland, A. M. Weinberg, R. M. Rotty, P. 

R. Bell, L. J. Allison, and C. L. Cooper (1982) . The carbon dioxide 

question; a perspective for 1982. In Clark (1982). 
Climate Research Board (1979) . Carbon Dioxide and Climate; A 

Scientific Assessment. National Research Council, National Academy 

of Sciences, Washington, D.C., 22 pp. 
C0 2 /Climate Review Panel (1982) . Carbon Dioxide and Climate; A 

Second Assessment. National Research Council, National Academy 

Press, Washington, D.C., 72 pp. 
Dickson, R. R., and J. Posey (1967). Maps of snow-cover probability 

for the northern hemisphere. Mon. Wea. Rev. 95; 347-53. 
Donner, L., and V. Ramanathan (1980). Methane and nitrous oxide: 

their effects on terrestrial climate. J. Atmos. Sci. 37:119-124. 
Eddy, J. A. (1976) . The Maunder minimum. Science 192:1189-1202. 
Eddy, J. A. (1977) . Climate and the changing sun. Climatic Change 

1:173-190. 



375 

Eddy, J. A., R. L. Gill Hand, and D. V. Hoyt (1982). Changes in the 

solar constant and climatic events. Nature 300;689-693. 
Elliott, W. P. (1982) . A note on the historical industrial production 

of carbon dioxide. NOAA Air Resources Laboratories, Rockville, Md. 
Ellis, J. S., T. H. Vonder Haar, S. Levitus, and A. H. Oort (1978). 

The annual variation in the global heat balance of the earth. J^ 

Geophys. Res. 83; 1958-1962. 
Emery, K. 0. (1980) . Relative sea level from tide gauge records. 

Proc. Nat. Acad. Sci. 77:6968-6972, 
Epstein, E. S. (1982). Detecting climate change. J. Appl Meteor ol. 

21:1172. 
Fels, S. B., J. D. Mahlman, M. D. Schwarzkopf, and R. W. Sinclair 

(1980) . Stratospheric sensitivity to perturbations in ozone and 

carbon dioxide: radiative and dynamical response. J. Atmos. Sci. 

37,: 2265. 
Fine, R. A., J. L. Reid, and H. G. Ostlund (1981). Circulation of 

tritium in the Pacific Ocean. J. Phys. Oceanog. 11:3* 
Fishman, J., V. Ramanathan, P. J. Crutzen, and S. C. Liu (1979). 

Tropospheric ozone and climate. Nature 282:818-820. 

Flint, R. F. (1971) . Glacial and Quarter nary Geology. Wiley, New York. 
Gates, W. L., K. H. Cook, and M. E. Schlesinger (1981). Preliminary 

analysis of experiments on the climatic effects of increased C(>2 

with an atmospheric general circulation model and a climatological 

ocean model. J. Geophys. Res. 86:6385. 
Geller, M. A., and V. C. Alpert (1980). Planetary wave coupling 

between the troposphere and the middle atmosphere as a possible 

sun-weather mechanism. J. Atmos. Sci. 37:1197-1215. 
Geophysics Study Committee (1982) . Solar Variability, Weather, and 

Climate. National Research Council, National Academy Press, 

Washington, D.C., 106 pp. 
Gilliland, R. L. (1981) . Solar radius variations over the past 265 

years . Astrophys. J. 248; 1144-1155 . 
Gilliland, R. L. (1982). Solar, volcanic, and C0 2 forcing of recent 

climatic changes. Climatic Change 4:111-131. 
Hammer, C. U. (1977). Past volcanism revealed by Greenland ice sheet 

impurities. Nature 270:482-486. 
Hansen, J. (1980). C0 2 , solar variations, recent climate, and model 

predictions. In U.S. Department of Energy (1980), pp. 144-153. 
Hansen, J., D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and G. 

Russell (1981) . Climate impact of increasing atmospheric carbon 

dioxide. Science 213:957-966. 
Hayashi, Y. (1982). Confidence intervals of a climatic signal, j^ 

Atmos. Sci. 39; 1895 . 
Hays, J. D. (1978). A review of the late quarternary climatic history 

of Antarctic seas. In Antarctic Glacial History and World 

Paleoenvironments ; August 17, 1977, Birmingham, United Kingdom. F. 

M. van Zinderen Bakker, ed. A. A. Balkema, Rotterdam, The 

Netherlands, pp. 57-71. 
Heath, D. F., and B. M. Schlesinger (1982). Secular and periodic 

variations in stratospheric ozone from satellite 

observations 1970-1979. NASA/Goddard Space Flight Center Preprint. 



376 

Hirschboeck, K. (1980) . A new worldwide chronology of volcanic 

eruptions . Palaeogeogr* Palaeoclim. Palaeoecol. 29; 223-241. 
Hoffert, M. I., A. J. Callegari, and C.-T. Hsieh (1980). The role of 

deep sea heat storage in the secular response to climatic forcing. 

J. Geophys. Res. 85; 6667. 
Hollin r J- T., and R. G. Barry (1979). Empirical and theoretical 

evidence concerning the response of the earth's ice and snow cover 

to a global temperature increase. Environ. Int. 2: 437-444. 
Hoyt f D. V. (1979a) . An empirical determination of the heating of the 

earth by the carbon dioxide greenhouse effect. Nature 282:388-390. 
Hoyt r D. V. (1979b) . Variations in sunspot structure and climate. 

Climatic Change 2; 79-92. 
Jones, P. D., T. M. L. Wigley f and P. M. Kelly (1982). Variations in 

surface air temperatures; Part 1. Northern hemisphere/ 1881-1980. 

Mon. Wea. Rev. 110; 59-70. 
Katz, R. W. (1980). Statistical evaluation of climate experiments with 

general circulation models; inferences about means. Climatic 

Research Institute Report 15. Oregon State U*, Corvallis. 
Katz, R. W. (1982) . Statistical evaluation of climate experiments with 

general circulation models; a parametric time series modeling 

approach. J. Atmos. Sci. 39; 1446-1455. 
Kelley, P. M. r and P. D. Jones (1981). Annual temperatures in the 

Arctic, 1881-1981. Climate Monitor 10; 122-124. 
Klein, W. H. (1982) . Detecting climate effects of increasing 

atmospheric carbon dioxide. In U.S. Department of Energy (1982) , 

pp. 175-194. 
Kraus, E. B. (1955) . Secular changes of tropical rainfall regimes. Q. 

J. Roy. Meteorol. Soc. 81: 108 . 
Kukla, G. J. (1981) . Climatic role of snow covers. In Sea Level/ Ice 

and Climatic Change, I. Allison, ed. International Assoc. of 

Hydrological Sciences Publ. 131. IAHS, Wallingford, Oxfordshire, 

U.K., pp. 79-107. 
Kukla, G. J. (1982) . Carbon dioxide and polar climates. In U.S. 

Department of Energy (1982), pp. 237-288. 
Lac is, A., J. Hansen, P. Lee, T. Mitchell, and S. Lebedeff (1981). 

Greenhouse effect of trace gases, 1970-1980. Geophys. Res. Lett. 

1:1035-1038. 
Lamb, H. H. (1970) . Volcanic dust in the atmosphere with a chronology 

and assessment of its meteorological significance. Phil. Trans. 

Roy. Phil. Soc. London; 425-533. 
Levy, H., J. 0. Mahlman, and W. J. Moxim (1979). A preliminary report 

on the numerical simulation of the three-dimensional structure and 

variability of atmospheric N 2 0. Geophys. Res. Lett. 6; 155-15 8. 
MacCracken, M. C. (1983) . Have we detected CO 2 -induced climate 

change? Problems and prospects. In U.S. Department of Energy 

(1983), Vol. 5, pp. 3-45. 
MacCracken, M. C., and H. Moses (1982). The first detection of carbon 

dioxide effects: workshop summary. Bull. Am. Meteorol. Soc. 

6.3:1164-1178. 
Madden, R. A., and V. Ramanathan (1980). Detecting climate change due 

to increasing carbon dioxide. Science 209; 763-68. 



377 

Manabe, S., and D. G. Hahn (1981). Simulation of atmospheric vari- 
ability. Mon. Wea. Rev. 109: 2260-2286 . 

Manabe, S., and R. J. Stouffer (1980). Sensitivity of a global climate 
model to an increase of C0 2 concentration in the atmosphere. J 
GeoPhVS. Res. 85; 5529-5554. pnere. ^ 

Manabe, S., and*. P. Strickler (1964). Thermal equilibrium of the 

atmosphere with a convective adjustment. J. Atmos. Sci. 21; 361-385. 
Manabe/ S., and R. T. Wetherald (1975). The effects of doubling the 

CO 2 concentration on the climate of a general circulation model. 

J. Atmos. Sci. 32;3. 
Manabe, S. r and R. T. Wetherald (1980). On the distribution of climate 

change resulting from an increase in C0 2 content of the atmosphere. 

J. Atmos. Sci. 37;99. 
Marland, G. , and R. Rotty (1982). Carbon dioxide emissions from fossil 

fuels, 1950-1981. Institute for Energy Analysis, Oak Ridge, Tenn. 
Mass, C., and S. H. Schneider (1977). Statistical evidence on the 

influence of sunspots and volcanic dust on long-term temperature 

records . J. Atmos. Sci. 34; 1995-2004 . 
Matson, M. , and D. R. Wiesnet (1981). New data base for climate 

studies. Nature 289; 451-456. 
Maxwell, J. B. (1980). The Climate of the Canadian Arctic Islands and 

Adjacent Waters. Environment Canada, Canadian Government Publishing 

Centre, Hull, Quebec, 531 pp. 
McCormick, M. P., W. P. Chu, L. R. McMaster, G. W. Grams, B. M. Herman, 

T. J. Pepin, P. B. Russell, and T. J. Swissler (1981) . SAM-II 

aerosol profile measurements, Poker Flat, Alaska, July 16-19, 1979. 

Geophys. Res. Lett. 8; 3- 4 
Miles, M. K., and P. B. Gildersleeves (1978). A statistical study of 

the likely influence of some causative factors on the temperature 

changes since 1665. Meteorol. Mag. 107; 193-204.. 
Mitchell, J. M. , Jr. (1961). Recent secular changes of global 

temperature . Ann. N.Y. Acad. Sci. 95; 235-250 . 
Mitchell, J. M. , Jr. (1970). A preliminary evaluation of atmospheric 

pollution as a cause of the global temperature fluctuation of the 

past century. In Global Effects of Environmental Pollution, S. F. 

Singer, ed. Springer-Verlag, New York, pp. 139-155. 
Mitchell, J. M., Jr. (1983). An empirical modeling assessment of 

volcanic and carbon dioxide effects on global scale temperature. 

American Meteorological Society, Second Conference on Climate 

Variations, New Orleans, Louisiana, January 10-14, 1983. 
Munk, W, and C. Wunsch (1982). Observing the ocean in the 1990s. 

Phil. Trans. Roy. Soc. London Ser. A 307; 439-464. 
Murphy, A. H. , and R. W. Katz (1982). Statistical methodology for 

first detection of carbon dioxide effects in the atmosphere. In 

U.S. Department of Energy (1982), pp. 165-174. 
Newell, R. E., and A. Deepak, eds. (1982). Mount St. Helens Eruptions 

ofW80--Atmospheric Effects and Potential Climatic Impact; NASA 

report SP-458. National Aeronautics and Space Administration, 

Washington, D.C. 




378 

Newhall, C. G., and S. Self (1982). The volcanic explosivity index 

(VEI) : an estimate of explosive magnitude for historical 

volcanism. J. Geophys. Res. 87; 1231-1238 . 
Oliver f R. C. (1976). On the response of hemispheric mean temperature 

to stratospheric dust: an empirical approach. J. Appl. Meteorol. 

15; 933-950. 
Oort f A. H. (1982). Global atmospheric circulation studies, 1958-1973. 

NOAA Prof. Paper. U.S. Govt. Printing Office/ Washington, D.C. (in 

preparation) . 
Oort, A. H., and E. M. Rasmusson (1971). Atmospheric circulation 

statistics. NOAA Prof. Paper 5. U.S. Govt. Printing Office, 

Washington, D.C. 
Paltridge, G., and S. Woodruff (1981). Changes in global surface 

temperature from 1880 to 1977 derived from historical records of sea 

surface temperature. Mon. Wea. Rev. 109;2427-2434. 
Pierotti, D., and R. A. Rasmussen (1977). The atmospheric distribution 

of nitrous oxide. J. Geophys. Res. 37; 5823-5832. 
Pierotti, D., and R. A. Rasmussen (1978). Inter-laboratory calibration 

of atmospheric nitrous oxide measurements. Geophys. Res. Lett. 

5;. 353-355. 
Pivavarova, Z. I. (1968). The long-term variation of the intensity of 

solar radiation according to the observations of actinemetric 

stations [in Russian] . Glavnaya Geofiz. Obs. Trudy 233; 17-37. 
Pivavarova, Z. I. (1977). Radiation Characteristics of the Climate of 

the USSR (in Russian) . Gidrometeoizdat, Leningrad. 
Plass, G. N. (1953). Some problems in atmospheric radiation. Proc. 

Toronto Meteorol. Conf.;53. Royal Meteorological Society, London. 
Plass, G. N. (1956). The carbon dioxide theory of climatic change. 

Tellus 8; 140-154. 
Pollack, J. B., 0. Toon, C. Sagan, A. Summers, B. Baldwin, and W. Van 

Camp, (1976a) . Volcanic explosion and climatic change; a research 

assessment. J. Geophys. Res. 81; 1071-1083. 
Pollack, J. B., 0. B. Toon, A. Summers, B. Baldwin, C. Sagan, and W. 

Van Camp, (1976b) . Stratospheric aerosols and climate change. 

Nature 263; 5578 
Potter, J. G. (1965). Snow cover. Climatological Studies 3. 

Meteorological Branch, Dept. of Transport, Toronto, Canada. 
Potter, G. L., H. W. Ellsaesser, M. C. MacCracken, and F. M. Luther 

(1975) . Possible climatic impact of tropical deforestation. Nature 

258;697-698. 
Potter, G. L., H. W. Ellsaesser, M. C. MacCracken, J. S. Ellis, and F. 

M. Luther (1980) . Climate change due to anthropogenic surface 

albedo modification. In Interactions of Energy and Climate, W. 

Bach, J. Pankrath, and J. Williams, eds. Reidel, Dordrecht, pp. 

317-326. 
Prabhakara, C., H. D. Chang, and A. T. C. Chang (1982). Remote sensing 

of precipitable water over the oceans from Nimbus 7 microwave 

measurements. J. Atmos. Sci. 59. 
Pritchard, R. S., ed. (1980). Sea Ice Processes and Models. 

Proceedings of the Arctic Ice Dynamics Joint Experiment, U. of 

Washington Press, Seattle, 474 pp. 



379 

Ramanathan, V. (1975). Greenhouse effect due to chlorof luorocarbons : 

climatic implications. Science 190; 50, 
Ramanathan, V. (1980) . Climatic effects of anthropogenic trace gases 

In Interactions of Energy and Climate, w. Bach, J. Pankrath, and j] 

Williams, eds. Reidel, Boston, Mass., pp. 269-280. 
Ramanathan, V. (1981) . The role of ocean-atmosphere interactions in 

the CO 2 -climate problem. J. Atmos. Sci. 38;918. 
Ramanathan, V., L. B. Callis, and R. E. Boughner (1976). Sensitivity 

of surface temperature and atmospheric temperature to perturbations 

in the stratospheric concentration of ozone and nitrogen dioxide. 

J, Atmos. Sci. 33;1092-1112. 
Rasmussen, R. A., and M. A. K. Khalil (1981). Increase in the 

concentration of atmospheric methane. Atmos. Environ. 15; 883. 
Reinsel, G. C. (1981) . Analysis of total ozone data for the detection 

of recent trends and the effects of nuclear testing during the 

1960s. Geophys. Res. Lett. 8; 1227-1230. 
Reinsel, J. C., J. C. Tiso, A. J. Miller, C. L. Mateer, J. J. DeLuisi, 

and J. E. Frederick (1983). Analysis of upper stratospheric Umkehr 

ozone profile data for trends and the effects of stratospheric 

aerosols. American Geophysical Union Spring Meeting, Baltimore, 

Maryland, May 30-June 3, 1983. Abstract in EOS 64; 199. 
Revelle, R., and H. E. Suess (1957). CC>2 exchange between atmosphere 

and ocean, and the question of an increase of atmospheric C0 2 

during the past decades. Tellus 9; 18. 
Robin, G. de Q., D. J. Drewry, and D. T. Meldrum (1977). International 

studies of ice sheet and bedrock. Phil. Trans. Roy. Soc. London 

Ser. B 279;185-96. 
Robock, A. (1978). Internally and externally caused climate change. 

J. Atmos. Sci. 35; 1111-1122 . 
Robock, A. (1979a) . The performance of a seasonal global climatic 

model. Report of the JOC Study Conference on Climate Models; 

Performance, Intercomparison, and Sensitivity Studies, Vol. 2. GARP 

Publ. 22. World Meteorological Organization, Geneva, Switzerland, 

pp. 766-802. 
Robock, A. (1979b) . The Little Ice Age; the northern hemisphere 

average observations and model calculations. Science 206; 1402-1404. 
Robock, A. (1981). The Mount St. Helens volcanic eruption of 18 May 

1980; minimal climatic effect. Science 212; 1383-1384. 
Robock, A. (1983). Ice and snow feedbacks and the latitudinal and 

seasonal distribution of climate sensitivity. J. Atmos. Sci. 

Ros^kr!nz? 9 p'. W., M. J. Komichak, and D. H. Staelin W* 2 \***^ 
for estimation of atmospheric water vapor profiles by microwave 
radiometry. J. Appl. Meteorol. 21; 1364. alho/ q 

Sagan, C., 0. B. Toon, J. B. Pollack (1979) . ^W albedo 
changes and the earth's climate. Science^: 1363-1367. 

St. John, D. S., S. P. Bailey, W. H. Fellner, J. M. *"X'j 
Snee (1981). Time series search for a trend in total ozone 
measurements. J. Geophys. Res. 86_x 7299-7311. Critical 

SCEP (1970). Man's Impact on the Global Environment, Study of Critical 
Environmental Problems/ MIT Press, Cambridge, Mass. 



1 



380 

Schlesinger, M. E. (1982). A Review of Climate Model Simulations of 

CQ ? - Induced Climatic Change . Report No. 41 , Climatic Research 

Institute, Oregon State U. f Corvallis f 135 pp. 
Schlesinger, M. E. (1983). Simulating C0 2 -induced climate change 

with mathematical climate models: capabilities/ limitations, and 

prospects. In U.S. Department of Energy (1983), III.3-III.140. 
Schneider, S. H., and S. L. Thompson (1981). Atmospheric C0 2 and 

climate: importance of the transient response. J. Geophys. Res. 

.86:3135. 
SchSnwiese, C. D. (1981) . Solar activity, volcanic dust, and 

temperature: statistical relationships since 1160 A.D. Arch. 

Meteorol. Geophys. Bioclimatol. Ser. A 30; 1-22 . 
Simkin, T., L. Seibert, L. McClelland, W. G. Nelson, D. Bridge, C. G. 

Newhall, and J. Latter (1981) . Volcanoes of the World. Hutchinson, 

Ross, New York. 
Siquig, R. A., and D. V. Hoyt (1980). Sunspot structure and the 

climate of the last one hundred years. In Ancient Sun; Proceedings 

of the Conference on the Ancient Sun: Fossil Record in the Earth, 

Moon, and Meteorites. Boulder, Colorado, Oct. 16-19, 1979. R. 0. 

Pepin, J. A. Eddy, and R. B. Merrill, eds. Pergamon, Elmsford, 

N.Y., pp. 63-67. 
Skoog, B. G., J. I. H. Askne, and G. Elgered (1982). Experimental 

determination of water vapor profiles from ground-based measurements 

at 21.0 and 31.4 GHz. J. Appl. Meteorol. 21:394. 
SMIC (1971) . Inadvertent Climate Modif ication f Study of Man's Impact 

on Climate. MIT Press, Cambridge, Mass. 
Stephens, G., G. Campbell, and T. Vonder Haar (1981). Earth radiation 

budgets. J. Geophys Res. 86:9739-9760. 
Taylor, R. A. H. (1980) . Snow Survey of the Northern Hemisphere. 

Glaciological Data, Report GD-9. World Data Center-A for Glaciology, 

Boulder, Colo., pp. 75-76. 
Thompson, S. L., and S. H. Schneider (1979), A seasonal zonal energy 

balance climate model with an interactive lower layer. J. Geophys. 

Res. 84:2401-2404. 
Thompson, S. L., and S. H. Schneider (1982). Carbon dioxide and 

climate: the importance of realistic geography in estimating the 

transient temperature response. Science 217:1031-1033. 
U.S. Department of Energy (1980) . Proceedings of the Carbon Dioxide 

and Climate Research Conference. Washington, D.C., April 24-25, 

1980. Prepared by The Institute for Energy Analysis, Oak Ridge 

Associated Universities, L. E. Schmitt, ed., Report No. 

CONF-80Q4110, UC-11, December 1980, 287 pp. 
U.S. Department of Energy (1982). Proceedings of the Workshop on First 

Detection of Carbon Dioxide Effects. Harper's Ferry, West Virginia, 

June 8-10, 1981. Harry Moses and Michael C. MacCracken, 

Coordinators. Prepared by The Institute for Energy Analysis, Oak 

Ridge Associated Universities, N. B. Beatty, ed., Report No. 

DOE/CONF-8106214, UC-11, May 1982, 546 pp. 
U.S. Department of Energy (1983) . Proceedings: Carbon Dioxide 

Research Conference: Carbon Dioxide, Science and Consensus. 

September 19-23, 1982. Berkeley Springs, West Virginia. Compiled 



381 

by The Institute for Energy Analysis/ Oak Ridge Associated 

Universities. Report No. CONF-820970, 506 pp. 
Vilmundarson, T. (1972) . Evaluation of historical sources of sea ice 

near Iceland. In Sea leer Proceedings of an International 

Conference at Reykjavik, Iceland, May 1971, pp. 159-169. 
Vinnikov, K. Ya. , and P. Ya. Groisman (1981). The empirical analysis 

of CO 2 influence on the modern changes of the mean annual Northern 

Hemisphere surface air temperature. Meteorol. Gidrol. 11:35-45. 
Vinnikov, K. Ya., and P. Ya. Groisman (1982). The empirical study of 

climate sensitivity. Atmos. Oceanic Phys. 18 (in press) . 
Vinnikov, K. Ya., G. V. Gruza, V. F. Zakharov, A. A. Kirillov, N. P. 

Kovyneva, and E. Ya. Ran'kova (1980). Current climatic changes in 

the Northern Hemisphere. Meteorol. Gidrol. 6; 5-17. (English 

translation by Allerton Press: Sov. Meteorol. Hydrol. 6:1-10.) 
Vinnikov, K. Ya., G. V. Gruza, V. F. Zakharov, A. A. Kirillov, N. P. 

Kovyneva, and E. Ya. Ran'kova (1981). Modern changes in climate of 

the northern hemisphere. Meteorol. Gidrol. 6:5-17. 
Walsh, J. E., and J. E. Sater (1981). Monthly and seasonal variability 

in the ocean- ice-atmosphere systems of the North Pacific and North 

Atlantic. J. Geophys. Res. 86:7425-7445. 
Wang, W. C. , Y. L. Yung, A. A. Lacis, T. Mo, and J. E. Hansen (1976). 

Greenhouse effects due to man-made perturbations of trace gases. 

Science 194; 685 . 
Weiss, R. F. (1981) . The temporal and spatial distribution of 

tropospheric nitrous oxide. J. Geophys. Res. 86:7185-7195. 
Weller, G., and B. Holmgren (1974). The microclimates of the Arctic 

tundra. J. Appl. Meteorol. 13:854-62. 
Wetherald, R. T., and S. Manabe (1981). Influence of seasonal 

variation upon the sensitivity of a model climate. J. Geoptr s. Res. 

16:1194. 

Wigley, T. M. L., and P. D. Jones (1981). Detecting C0 2 -induced 

climatic change . Nature 292; 205-208 . 
Willett, H. C. (1950) . Temperature trends of the past century. 

Centenary Proceedings 195 , Royal Meteorological Society. 
Willett, H. C. (1974a) . Recent statistical evidence in support of the 

predictive influence of solar-climate cycles. Mon. Wea. Rev* 

102:679-686. 
Willett, H. C. (1974b) . Do recent climate fluctuations portend an 

imminent ice age? Geofis. Intern. 14:265-302. 
Willson, R. C., S. Gulkis, M. Janssen, H. S. Hudson, and G. A. Chapman 

(1981) . Observations of solar ir radiance variability. Science 

211:700-702. 
World Meteorological Organization (1981) . Joint WMO/ICSU/UNEP Meeting 

of Experts on the Assessment of the Role of CO? on Climatic 

Variations and Their Impact (Villach, Austria, November 1980) . 

World Climate Programme Report No. 3. World Meteorological 

Organization, Geneva, 35 pp. 
World Meteorological Organization (1982a) . Report of the WMO(CAS)/JCS 

Meeting of Experts on Detection of Possible Climate Change (Moscow, 

October 3-6, 1982) . World Climate Programme Report No* 29. World 

Meteorological Organization, Geneva, 43 pp. 



382 

World Meteorological Organization (1982b) . The International Satellite 

Cloud Climatology Project (ISCCP) Preliminary Implementation Plan. 

World Climate Programme Report No. 35. World Meteorological 

Organization, Geneva, 85 pp. 
Wuebbles, D. J., F. M. Luther, and J. E. Penner (1983). Effect of 

coupled anthropogenic perturbations on stratospheric ozone. J. 

Geophy. Res. 88; 1444-1456 . 
Yamamoto, R. (1980) . Change of global climate during recent 100 

years. Proceedings of the Technical Conference on Climate Asia and 

Western Pacific, Guangzhou, China, 15-20 Dec. 1980. World 

Meteorological Organization, Geneva, pp. 360-375. 
Zwally, H. J., R. H. Thomas, and R. A. Bindschadler (1981). Ice-sheet 

Dynamics by Satellite Laser Altimetry. Technical Memorandum 82128. 

National Aeronautics and Space Administration, Goddard Space Flight 

Center, Greenbelt, Md., May 1981, 11 pp. 
Zwally, H. J., R. A. Bindschadler, A. C. Brenner, T. V. Martin, and R. 

H. Thomas (1983a) . Surface elevation contours of Greenland and 

Antarctic ice sheets. J. Geophys. Res. 88;1589-1596. 
Zwally, H. J., C. L. Parkinson, and J. C. Comisco (1983b) . Variability 

of antarctic sea ice and C02 change. Science 220; 1005-1012. 



Agriculture and a Climate 
6 Changed by More Carbon Dioxide 



PaulE. Waggoner 



6.1 INTRODUCTION 

The crops that feed us stand outdoors in the wind, rain, and frost. 
Except for 17 pounds of fish in the 1400 pounds each American eats 
yearly, all the food for us and the feed for our animals, too, grows on 
a third of a billion acres of cropland and vast rangelands and pas- 
tures, exposed to the annual lottery of the weather. Although about 50 
million acres of American crops are protected from drought by irriga- 
tion, even these depend on precipitation in the long run, and sheltering 
crops in greenhouses from temperature extremes is too expensive for 
staples. Thus, decade after decade grocery prices for all and hunger 
of the poor puncture the arrogance of our technology and remind us that 
a few chance degrees of warmth or drops of rain, properly timed, protect 
us. If it were not so, "Then all the concern about future climate that 
has been so widespread in recent years would be much ado about nothing" 
(McQuigg, 1979) . 



6.1.1 Concentrating on a Critical, Susceptible, and Exemplary Subject 

Agriculture is broad. It is the science and art of the production of 
plants and animals useful to man and in varying degrees their prepara- 
tion for man's use and their disposal by marketing. This chapter on 
agriculture could, therefore, encompass forests and fisheries as well 
as farmers, hunters, and grocers as well as farmers, logs, and hogs 



Several individuals have assisted the author. Clarence Sakamoto, 
Norton Strommen, and Tom Hodges assembled the data and used the 
regression and simulator models to predict changes in yield. Herbert 
Enoch, Gary Heichel, Robert Loorais, Israel Zelitch, and James Tavares 
contributed to the section on the effects of CO 2 on photosynthesis 
and plant growth. Marvin Jensen and Glenn Burton advised on water and 
breeding. The author gratefully acknowledges these contributions, but 
it is the author alone who takes responsibility for any errors or 
omissions. 

383 



384 

as well as crops. It could encompass the Ukraine and the Humboldt 
current, Iowa and the Hoboken docks, California and the Amazonian 
forest. Although weather surely affects all of these, their survey in 
a single chapter would be incomplete or superficial, and we have 
concentrated on a critical, susceptible, and exemplary part; American 
crop production. 
*' Crops are critical because without their photosynthetic conversion 

of solar to food energy there would be neither bread nor meat. American 
crops are critical to Americans because they feed us and bring in $40 
billion of our foreign exchange (USDA, 1982) . And, they are critical 
to others: for example, in 1979 the United States provided 42% of the 
wheat and 19% of the rice exported by the nations of the world, and 
fully 43% of the world's corn crop is American (USDA, 1982). 

The susceptibility of crops, rooted in place and exposed outdoors, 
makes them biologic indicators of weather. American crops growing from 
35 to 49 degrees latitude are within the zone that meteorologists 
predict will experience a change in the weather as C0 2 increases, and 
thus these critical and susceptible crops are also exemplary. Often we 
shall concentrate further, examining wheat, corn, and soybeans, which 
outdistance in value any other American crop. 

6.1.2 Agriculture and Past Changes in the Weather 

ii 

Later paragraphs will show technical calculations and projections of 
changes in American crops matching projections of the weather. Some 
were derived from historical statistics. Changes in the weather cause 
changes in agriculture that are too complex to be distilled into a few 
statistics, however, and at this point a background of real life stories 
is painted. History is, after all, the laboratory notebook where the 
results of experiments performed by Nature herself are recorded. 

Nature has not, of course, actually experimented less than 100 ppm 
with C0 2 within the human era. Nevertheless, her experiments with 
temperature, rain, and snow show in general how farmers are affected by 
atmospheric change, and then C02 may bring changes in temperature and 
water themselves. 

When rain fell abundantly on Italy from 450 to 250 B.C., intensive 
agriculture was fruitful and the Roman Republic was vigorous. From 100 
B.C. to A.D. 50 rain again fell abundantly and Roman civilization was 
high. After A.D. 80 rain was light, vines and olives replaced cereal, 
and the Roman Empire declined and fell (Brooks, 1970) . This demon- 
strated the adaptation of agriculture by changing crops but the new 
crop did not sustain the Empire. 

The years 1301-1350 experienced a change in climate, and Brooks 
(1970) calculated a maximum of raininess for the half century. Tuchman 
(1978) wrote that the medieval people were unaware that 

...owing to the climatic change, communication with Greenland 
was gradually lost, that the Norse settlements there were being 
extinguished, that cultivation of grain was disappearing from 
Iceland and being severely reduced in Scandinavia. But they 



385 

could feel the cold, and mark with fear its result: a shorter 
growing season. 

This meant disaster, for population increase in the last 
century had already reached a delicate balance with agricul- 
tural techniques. Given the tools and methods of the time, the 
clearing of productive land had already been pushed to its 
limits. Without adequate irrigation and fertilizers, crop 
yield could not be raised nor poor soils be made productive. 
Commerce was not equipped to transport grain in bulk from 
surplus-producing areas except by water. Inland towns and 
cities lived on local resources, and when these dwindled, the 
inhabitants starved. 

In 1315, after rains so incessant that they were compared to 
the Biblical flood, crops failed all over Europe, and famine, 
the dark horseman of the Apocalypse, became familiar to all. 
The previous rise in population had already exceeded agricul- 
tural production, leaving people undernourished and more 
vulnerable to hunger and disease. Reports spread of people 
eating their own children, of the poor in Poland feeding on 
hanged bodies taken down from the gibbet. 

In this ancient experiment were demonstrations that colder as well 
as warmer weather can damage; also the length of the season as well as 
the mean temperature is critical, transportation can alleviate hunger, 
and the impact of changed climate is sharp at the poleward margin of 
farming. 

In 1886-1893 an American experiment was performed in the Middle 
Border, and the Wayne Township (Kansas) Farmer's Club recorded the 
results (Malin, 1936) . Problems were met with full force because 
movement into the region was swift during the most favorable weather of 
30 years. After the farmers were settled, however, "Out of 471 acres 
of fall wheat there is not wheat enough to cover 15 acres. All winter 
killed." Although flooding was sometimes a problem, drought was the 
major force. In 1886 Turkey wheat was not favored because millers did 
not like it, but 4 years of hot summers and cold winters made the hardy 
Turkey wheat the favorite. Although in 1885 the acreage of corn per 
farm was four times that of wheat, by 1905 there were more than 3 acres 
of wheat for every acre of corn. The predominance of grazing land over 
cropland ran in cycles with the weather. A final adaptation was 
flight: in 1895 a traveler across the County reported that it was 
practically deserted. Enough people were left, however, for the 
subject of "Rainfall and the Populist Party in Nebraska" (Barnhart, 

1925) 

The Dust Bowl of the 1930s was a natural experiment with results 
dramatized in John Steinbeck's Grapes of Wrath (1939), and drifting 
soil is vividly remembered. The catastrophe was not only wilting of 
crops: pests encouraged by the drought amplified the damage (National 
Research Council, 1976) . 



386 

The red spores of wheat rust were blown over the Wheat Belt and 
infected wheat; rust fungus erupted from stems and leaves. The greatest 
losses of wheat during 1921-1950 were in the 1930s: 3.4 million metric 
tons in 1935 and 1937 were much greater than the runner-up/ 1 million 
metric tons in 1923. 

Weeds invaded. The prickly pear cactus headed east, invading 1.6 
million hectares in western Kansas. More young jackrabbits survived in 
drier weather, and productive rangeland was turned into pastures of 
prickly pear with a plague of jackrabbits. 

Grasshopper plagues are generally associated with drought. Like a 
chapter in Exodus, the report of 1936 was of a loss of $106 million, 
more than the total gross income from all farm products in Arizona, 
Nevada, New Mexico, Utah, and Wyoming combined. The hoppers made such 
a clean sweep in South Dakota that jackrabbits, faced with starvation, 
escaped into Nebraska (Schlebecker , 1953). 

Finally, an experiment in direct effect on cattle is added. A 
cattle boom began in Dakota in 1883, The Bad Lands Cow Boy claimed > "We 
have never heard of a solitary head ever having died in the Bad Lands 
because of exposure," and Theodore Roosevelt invested. In 1886-1887 
storm piled on storm, children were lost and froze within a hundred 
yards of home, and cattle, desperate for shelter, smashed their heads 
through ranch house windows. The average loss of cattle was 75%, and 
Roosevelt rode for 3 days the following summer without seeing a live 
steer. Extreme weather rather than the average affected farming. A 
change of weather and the consequent boom to bust took only 3 or 4 
years (McCullough, 1981) . 

Nature's experiments demonstrated the following: 

Farmers fit husbandry and crops to the weather. 
Swift change disrupts. 

Colder as well as warmer and wetter as well as drier can 
damage . 

Pests amplify effects of bad weather. 
The very soil can be changed by weather. 

Even warm-blooded animals are affected by weather. 
Occasional extremes destroy agriculture. 

Impact of changed weather is sharp in marginal climates. 

Migrations and political upheaval are blown by bad weather. 

Empires rise as well as fall as yields wax and wane in 
changing climates. 

Against this background we shall now examine the range of change 
meteorologists set before us and then calculate or speculate from 
science on the changes in crops that would follow. 



6.1.3 The Range of Change in the Atmosphere 

Carbon Dioxide. The C0 2 concentration of the atmosphere has risen 
from below 300 parts per million by volume (ppmv) in the 1800s to about 
340 ppmv at present. It is estimated that by the year 2025 it will 



387 

likely reach 425 ppmv, and by 2065 it will probably pass 600 ppmv 
(Nordhaus and Yohe, this volume, Chapter 2, Section 2.1). Nordhaus and 
Yohe's upper estimate for C02 concentration in the year 2000 is 400 
ppmv (Chapter 2, Figure 2.23). We shall see that CO 2 has a direct 
effect on crops. 

Temperature. Available climate models indicate that a doubling of the 
C02 content could raise the global annual average surface temperature 
by 3C (this volume, Chapter 4). If we assume the rapid rise in 
atmospheric concentrations to 400 ppmv in A.D. 2000, the temperature 
rise by A.D. 2000 would be about 1C. In polar latitudes a doubling of 
the atmospheric C0 2 concentration would cause a 5 to 10 C warming 
(Mitchell, 1977) . The polar and higher latitudes are sensitive because 
of summer changes in the albedo of regions normally covered by snow and 
ice throughout the year. For middle latitudes, Manabe and Stouffer 
(1980) calculated a lesser warming: 3C at the U.S. -Canadian border 
after a doubling of C02. 

Length of Growing Season. Kellogg (1977) pointed out a simple relation 
between summertime mean temperature and the length of the growing 
season at middle and high latitudes: A 1C change in mean temperature 
for the summer corresponds approximately to a 10-day change in the 
length. We must beware that the predicted annual average temperatures 
include winter, and conceivable changes in daily amplitudes or vari- 
ability (Neild, 1979) could modify the simple relation. Using this 
10-day rule of thumb, however, one can reasonably consider the effect 
on crops of a lengthening of the growing season by 10 days in the 
northern United States where a 1C increase in average annual temper- 
ature has been predicted by A.D. 2000. 

Precipitation and Moisture. Manabe and his colleagues (1981) have 
projected the changes in soil moisture that will accompany the predicted 
rise in temperature with three models of increasing geographic detail. 
They estimated the change from present C02 concentrations to four times 
as much. The three models all predicted drier summers at middle and 
high latitudes. Snow would melt earlier at high latitudes, causing an 
earlier transition from spring to summer and less rain. The earlier 
onset of summer would evaporate and transport more moisture from the 
soil. 

Conclusion. Looking toward a horizon near A.D. 2000, the agricultural- 
ist may be uncertain how the climate of a precise place will change, 
but he can reasonably consider how crop production would be changed by 
an increase to about 400 ppmv, a mean warming of about 1C in the 
northern United States with a growing season about 10 days longer, and 
more frequent drought in the United States caused by somewhat less rain 
and slightly more evaporation. 



1 



38S 
6.2 EFFECTS OF C0 2 ON PHOTOSYNTHESIS AND PLANT GROWTH 

Carbon dioxide is a major substrate for photosynthesis and, therefore, 
can directly affect plant growth if C0 2 is limiting. The current 340 
ppmv appears to be limiting (Figure 6.1) and, therefore, a rise in 
atmospheric C0 2 levels should increase photosynthesis. However, most 
effects of CO^ on photosynthesis and plant growth have been studied 
and measured during short periods where other factors such as light, 
water, temperature, and nutrients are optimal. In addition, growth 
habits and adaptations to different environments might mitigate or alter 
the effects of changing CO 2 concentration. Yield is the important 
integration of physiology, and it is difficult to predict the changes 
in yield that might follow a rise to 400 ppm of C0 2 . This section 
summarizes the current understanding of the direct effects of C0 2 on 
plant growth and the factors that must be considered in projecting 
these effects to yields. More discussion of direct effects can be 
found in the proceedings of the International Conference on Rising 
Atmospheric Carbon Dioxide and Plant Productivity, Athens, Georgia, May 
1982 (Lemon, 1983) . 

In addition to showing the direct effect of CO 2 on plants, the 
curves of Figure 6.1 are critical in calculating the rise in C0 
itself. The percent change in photosynthesis per percent change in 
C0 2 in the air, which is typically covered by the term "beta" in 
carbon cycle models (c.f. Woodwell, this volume, Section 3.3.3), must 
be about 0.25 to account for the rise caused by burning fossil fuel 
since the Industrial Revolution (Bjorkstrom, 1979). The beta of the 
illuminated wheat leaf in 400 ppmv C0 2 (Figure 6.1) is 0.8 but of the 
corn leaf is 0.1. These botanical estimates from the laboratory 
certainly do not conflict with the estimate of 0.25 from the global 
fuel consumption and atmospheric C0 2 . 

6.2.1 Photosynthes is 

About 90% of the dry weight of plants derives from the reduction of 
C0 2 to carbohydrates by photosynthesis. In single leaves in bright 
light, net photosynthesis increases with CO 2 above the current 
atmospheric level of 340 ppmv, and this is confirmed in whole plants by 
greater crop yields (see Section 6.2.6). 

On the basis of photosynthetic properties plants are classified into 
C 3 plants, C 4 plants, and crassulacean acid metabolism (CAM). Although 
photosynthesis occurs in all plants by the C 3 pathway, the C 4 and CAM 
plants have specialized steps to sequester C0 2 into the leaf. In C 4 
plants C0 2 is incorporated into C 4 -dicarboxylic acids that can be 
transported within the leaf to sites where C0 2 is released for photosyn- 
thesis. In specialized desert plants of the CAM type the pores in the 
leaves (stomates) open at night to collect C0 2 into organic acids that 
later regenerate the C0 2 inside the leaf for photosynthesis during 
daylight. This allows CAM plants to keep their stomates closed during 
the heat of daylight and thereby save water. 



389 



100 




200 



400 600 

C0 2 (ppmv) 



800 



FIGURE 6.1 Typical photosynthesis response of plants to C0 2 . Net 
photosynthesis of wheat is about 70 mg of CO 2 dm"" 2 h~^ compared with 
maize (about 55 mg of C0 2 dm"" 2 h" 1 ) for equivalent light intensity (0, 
cal cm"* 2 min"" 1 ) . Maize is saturated at a lower C0 2 concentration 
(-450 ppm) than wheat (-850 ppm) . CO 2 in ppmv is percent C0 2 x 10 4 . 
(Adapted from Akita and Moss, 1973.) 



The same enzyme that catalyzes the first step in the reduction of C0 2 
to carbohydrates can also oxidize the first product. This oxidation 
occurs in light also but uses O 2 to oxidize carbon back to CO 2 . It 
is called photorespiration/ and its rate is determined by the ratio of 
C0 2 to 2 within the leaf. Photorespiration rates are high in C 3 
plants. By increasing C0 2 levels from 340 to 400 ppmv, CO 2 uptake is 
enhanced about 20% in C 3 (high-photorespiration) species and about 7% in 
C 4 (low-photorespiration) species (Hesketh, 1963; Akita and Moss , 1973). 
Thus, increasing CO 2 directly benefits 3 species more than C 4 species. 
Rising C0 2 can be expected to alter leaf carbon metabolism and thereby 
affect the rate and duration of photosynthesis and the fate and par- 
titioning of the photosynthate . 



6.2.1.1 Rate of Photosynthesis 

The faster C0 2 fixation per leaf area at higher C0 2 may be explained by 
the kinetic properties of ribulose bisphosphate carboxylase, the primary 
C0 2 -fixing enzyme in the chloroplast, and to some extent by the con- 



390 

sequently slower photorespiration in C 3 species. The carboxylase is 
a large molecule (mol wt = 560,000) that comprises about half the total 
chloroplast protein and about a quarter of all the protein in the 
leaf. Half-maximal rates of C02 fixation with the isolated enzyme 
are obtained at about 600 ppmv of CO 2 (Jensen and Bahr, 1977) , which 
is not greatly difficult from the curvilinear increase in net photo- 
synthesis of leaves in rising C0 2 (Figure 6.1). Neither the photo- 
chemistry and electron transport nor the rate of regeneration of the 
C0 2 acceptor by the photosynthetic carbon reduction cycle appear to 
limit photosynthesis in the range of C0 2 under consideration. 

The release of photorespiratory C0 2 is faster at higher O 2 concen- 
trations t while increasing CO 2 levels inhibit photorespiration (Zelitch, 
1971) . The competition between CO 2 and 2 for photosynthesis and photo- 
respiration is expressed in C 3 species as an inhibition of photosyn- 
thesis by O 2 , or as an "oxygen stress" (Zelitch, 1982). A large propor- 
tion of the inhibition of photosynthesis by 2 is attributable to 
photorespiration, because losses of photorespiratory CO 2 can be as high 
as 50% of net photosynthesis in C 3 plants (Zelitch, 1982) . Photosyn- 
thesis in tobacco leaves was inhibited 35% at 340 ppmv of C0 2 at 21% 
of 2 compared with 3% of O 2 and was inhibited 31% at 400 ppmv of C0 2 
(R. B. Peterson, Connecticut Agricultural Experiment Station, New 
Haven, Connecticut, personal communication, 1982) . Thus, the benefit 
of regulated photorespiration appears less than the advantage of faster 
carboxylation when C0 2 is raised from 340 to 400 ppmv. 

6.2.1.2 Duration of Photosynthesis 

Prolonged and faster photosynthesis caused by increased C0 2 in bright 
light produces more sucrose, sometimes increasing starch accumulation 
in the chloroplasts, and excessive starch can deform chloroplasts and 
decrease photosynthesis (Guinn and Mauney, 1980) . Besides increasing 
sucrose, faster photosynthesis may change the levels of phosphorylated 
compounds and decrease orthophosphate levels in chloroplasts, which 
feed back to inhibit photosynthesis and decrease C0 2 assimilation 
(Walker, 1976). Such negative feedback might be bred against to obtain 
the full benefits of increased C0 2 on yield. 

Yield depends on an adequate storage or sink to accept the products 
of photosynthesis. If the sink is inadequate, feedback will decrease 
photosynthesis. Sink capacity and yield tend to increase in parallel 
until they reach the limit set by the photosynthetic capacity (Evans, 
1975) . 



6.2.1.3 Pate and Partitioning of Photosynthate 

The products of photosynthesis and the efficiency of their translocation 
to the sites of conversion to starch, protein, and lipids may affect 
photosynthesis itself as well as the accumulation of carbon in the 
storage organ. Higher C0 2 decreases photorespiration and thus 
indirectly will affect nitrogen metabolism, since ammonia turns over 



391 

rapidly in leaves during photorespiration, and the balance of amino 
acids available for storage will change (Lawyer et al., 1981). Although 
less lipids might be made in leaves/ more may be produced in storage 
organs that synthesize lipids from translocated sucrose. These specula- 
tions are based on sound biochemistry/ but data are lacking. 



6.2.2 Drought 

C0 2 directly affects the water in plants because C0 2 affects the pores 
or stomata through which water is transpired. The epidermis of leaves 
is generally perforated by countless microscopic stomata ? where the 
size of the opening is regulated by the shrinking and swelling of two 
guard cells that border it. The CO2 for photosynthesis is acquired 
through these pores , and since the pores open into the moist interior 
of the leaves, water is transpired. Most stomata close in the dark, 
conserving water when photosynthesis stops. In fact, mere narrowing of 
the pores conserves water, even in a crop with several acres of leaf 
surface area per acre of land (Waggoner et al., 1964). By regulating 
the size of stomata, plants can seek a balance between the necessary 
uptake of C02 and the stresses caused by excessive loss of water. We 
noted earlier the unique method used by CAM plants to acquire CO2 
under arid conditions. 

In bright light, maize stomata narrow when C0 2 concentration 
increases from 300 to 600 ppmv, and transpiration decreases about 15% 
from the potted plants. Transpiration from pots of wheat decreased 
only about 5%, however, and the difference between wheat and corn is 
assumed typical of the difference between 03 and C 4 plants (Akita 
and Moss, 1973). In crops in the field. Baker (1962, 1965) observed 
that transpiration of corn and cotton in bright light decreased by 20 
and 35% between 300 and 600 ppmv. 

The loss of water from the soil includes evaporation from the soil 
surface itself, especially when it is moist and unshaded by foliage, 
and from wet foliage as well as transpiration through stomata. Thus 
the loss of water from a field, month in and month out, will be affected 
less by C02 than by the percentages of 5 to 35% given above. 

Although we know of no experiments showing that crops can tolerate 
drought in CC>2-rich air, there is some foundation to expect an increase 
to 400 ppmv of CO 2 would decrease transpiration from a crop with 
abundant foliage by the order of 5%, somewhat more in 4 and somewhat 
less in C 3 crops. Since reduced transpiration would deplete soil water 
somewhat more slowly, drought would logically be somewhat less frequent. 



6.2.3 Nutrients 

The demand for nitrogen and mineral nutrients in plant growth is tied 
to photosynthesis. As photosynthesis increases with increasing CO 2 r 
the carbohydrate available for plant growth will increase and, in turn, 
impose demands for increased fertilizer or available soil nutrients. 
Since plant biomass typically has a minimum nitrogen content of between 



392 

2 and 3% of dry matter and a phosphorus content near 0.2 to 0.3% f nutri- 
ents may be needed to realize any increase in production. 

C0 2 may improve the availability of nutrients: Photosynthates are 
utilized as energy by symbiotic nitrogen-fixing organisms. The nitrogen 
in plants with such symbionts associated with their roots may increase. 
For the same reason more photosynthates might increase uptake of N, P, 
and K by roots in their association with bacteria and fungi. And 
finally, increased photosynthesis and growth could enlarge the pool of 
decomposing soil organic matter that can serve as a reservoir of soil 
nutrients. 



6.2.3.1 Nitrogen Metabolism 

Nitrogen fixation consumes much photosynthate for its energy. Nitrogen- 
fixing symbionts such as Rhizobium and Frank ia/ therefore, are greatly 
affected by the photosynthate available in the plant. While quantita- 
tive data are limited, experiments with CO-enriched plants show 
increased N2 fixation. In field experiments, Hardy and Havelka 
(1977) showed more N 2 fixed in soybeans in air enriched with C0 2 to 
800-1200 ppmv, and the total kilograms of N 2 fixed per hectare over 
the growing season was much higher with CO 2 -enrichment. In terms of 
harvestable product, Rogers et al. (1980) found a 28% increase in 
weight of seeds harvested per soybean plant at 520 ppmv of C0 2 in 
comparison with 340 ppmv. 

Of all the essential elements for plant growth it is usually 
nitrogen that is limiting. Thus the increased photosynthesis in a 
C0 2 -enriched atmosphere will increase demand for nitrogen. There 
also might be a shift to plants capable of N 2 fixation and to plants 
that have a beneficial association with free-living, N 2 -fixing microbes. 
The increased nitrogen in these plants would permit increased growth 
and, in turn, increased photosynthesis, more photosynthate, and even 
more N 2 could be fixed until some other factor became limiting. 

6.2.3.2 Organic Matter and Rhizosphere Association 

Increased atmospheric CO 2 will not likely affect the rhizosphere. 
The respiration of roots and soil microbes maintain CO 2 concentra- 
tions in the air spaces in soil that are 10 to 50 times higher than in 
the atmosphere. Doubling atmospheric C0 2 would likely have little 
direct effect on roots and soil microbes. 

If C0 2 increased plant growth it could increase the plant remains 
incorporated as soil organic matter and influence the cycling of min- 
erals in the soil and other soil properties. Soil organic matter is 
composed of two major factions that occur in roughly equal amounts. 
One factor is rapidly turning over and is composed of readily metabo- 
lized organic molecules such as cellulose; the other is composed of 
slowly accumulating, stabilized aggregates of phenols, other aromatic 
molecules, and inorganic particles with turnover times of at least 100 
years (Van Veen and Paul, 1981) . Although increases in degradable 



393 

biomass from green manuring generally raise the activity of soil 
microbes and change soil organic matter only slightly, C0 2 will be 
recycled to the atmosphere faster in response to the increase of 
degradable organic matter . With increased substrates in the form of 
root exudates or degradable soil organic matter there may be increased 
nitrogen fixation by bacteria, both free-living and symbiotic, and 
increased mycorrhiza. 

Many minerals required for growth, such as P, Zn, and Cu, are 
unavailable to roots because they are predominantly in immobile and 
insoluble forms in the soil. Micorrhizal fungi may increase their 
availability to host plants by penetrating more soil. 

Thus, increased photosynthesis and degradable biomass are likely to 
increase soil nitrogen levels, perhaps by 5 to 10%, and may slightly 
decrease the phosphorus and soil nutrients not tied up in living 
biomass (Lemon, 1983) . 



6.2.4 Phenology 

The development of new organs is distinct from their mere enlargement 
or growth, and one can reasonably ask if this development is affected 
by CC>2 in the air. 

Although one plant, cucumber, flowered earlier and the fruit was 
ready for market two weeks earlier when the air was enriched with 
CO 2 , another plant, pepper, proceeded at the same rate (Enoch et al., 
1970) . C0 2 did not change the period from pruning to harvest of 
roses (Zieslin et al., 1972), and a tenfold increase in C0 2 scarcely 
shortened the time from flowering to ripe strawberries (Enoch et al. , 

1976). 

Thus changed timing of flowering and other stages in the life of 
crops is not likely to be an important consequence of the rise in C0 2 
that we are considering. 



6.2.5 Weeds 

Environment affects crops, their pests, and the relation between them. 
Among the insect, disease, and weed pests of plants, it is the weeds 
fueled by their own photosynthesis that can be directly affected by 
changing atmospheric C0 2 . We shall examine the effects of ^*" 
and disease on crops later in this chapter in the context of changes in 



temperature and moisture . 

Annually weeds exact a toll of some $18 billion in the 
by competing with crops for light, water, and 
1981) . P Because C 3 and C 4 *^<^ 
atmospheric CO, concentrations, the generalities ot weea 
have been examined in those terms. Will the t " 



weed if it is C 3 and benefits from an i?2a2* 
while the crop is C 4 and responds less to increasing 

Twelve of the 15 crops that feed the world are C 3 P 1 **^ a 
1975) , whereas 14 of the 18 most damaging weeds are C 4 (Patterson, 



a 



*~'i 



394 

1982). Although that seems good fortune r 19 of the 38 major weeds of 
American maize/ a 04 plant f are 03 plants (USDA f 1972). 

The generality of Cj gaining relative to C^ plants in rising CC^ 
has been demonstrated by crops and weeds. The C 3 weed velvet leaf 
(Abutilon theophrasti) increased its growth more than maize when C0 2 
was increased. On the other hand, the C 4 weed, itch grass (Rottboellia 
exaltata) , gained less than the C 3 crop, soybean (Glysine max) 
(Patterson and Flint, 1980) . 

In addition to a gradual increase of a few percent in growth, C0 2 
can conceivably remove a limitation to the spread of a weed. Thus, 
okra, which is a crop becoming a weed, can grow at lower temperature 
and presumably at higher latitudes if CC>2 is enriched (Sionit et al., 
1981b) . . 

Limitations of fertilizer, water, or light might of course limit the 
realization of an advantage of CO2 to a weed or crop. In fact, how- 
ever, limited nutrients and water have failed to nullify the benefits 
of C02r including those to the height and leaf area that will affect 
competition for light (Patterson and Flint, 1982) . Thus increases in 
atmospheric C02 may affect the competition of weeds and crops, some- 
times to the advantage of the crop and sometimes to the weed. 



6.2.6 Direct Effects of COo on Yield 

The integration and the practical outcome of all the effects enumerated 
above is yield. Calculating the direct effect of increased C02 on 
yield is chancy because of lack of experimental data. Few food, feed, 
or fiber crops have been grown at elevated C02 from sowing to harvest. 
Most experiments have been brief, with emphasis on a specific stage of 
growth, and have been conducted with flowers and ornamentals rather 
than crops. Kimball (1982) recently summarized the results of 70 CC>2 
enrichment experiments conducted during the past 64 years. In Table 
6.1 the results of several experiments are presented for the entire 
life cycle of crops. 

All experiments were in growth chambers and greenhouses; no results 
from fields were available. In all experiments plants were grown in 
optimum environments without pests and with abundant water and nutri- 
ents. We cannot claim that light was optimum because plants seldom 
enjoy this status when grown in chambers and greenhouses. Although all 
experiments were performed in equable environments, all experiments 
were, nevertheless, dissimilar. Some experiments included stressful 
conditions as treatments. As these stresses might reflect a situation 
encountered in the field, their effect on response of yield to C0 2 
was important to our evaluation. 

First beta, here taken as the percent change in yield per percent 
change in CC>2, is examined. The range is a 0.1 to 0.9% increase in 
dry weight per percent increase in CO 2 . As in the estimation of beta 
from the simple response to C0 2 shown in Figure 6.1, the global 
estimate of 0.25 for beta (see Woodwell, this volume, Chapter 3, 
Section 3.3) is not denied by these additional botanical estimates. In 
addition, there is evidence in Table 6.1 that drought or lack of 



395 

TABLE 6.1 Changes in Yields of Crops in Optimum and Stressful 
Environments Anticipated from Atmospheric Enrichment to 400 ppmv of CO? 



Crop 



Change 

in 

Yield Component 

(%) Harvested 



Yield 

Increment/ Yield 
CC>2 Change by 

I nc r emen t Enr ichmen t 
(%/ppmv (%/60 ppmv 
of C0 2 ) of CO 2 ) 



Reference 



Optimum Environments 

Barley 0.9. 

Corn 0.28 

Cotton . 6 

Soybean . 4- 

Wheat 0.4 

Wheat . 3 

Wheat . 6 

Stressful Environments 



Grain 0.18 

Young shoots 0.03 

Lint 0.34 

Grain 0.04 



Grain 
Grain 
Grain 



0.13 
0.07 
0.13 



11 Gifford et al. (1973) 

1*9 Wong (1979) 

20 Mauney et al. (1978) 

2 Hardman and Brun 

(1971) 

8 Gifford (1979) 

4 Sionit et al. (1980) 

8 Sionit et al. (1981a) 



Corn 





.28 


Young shoots 





.03 


1.9 


Wong (1979) 


(1/3 normal N) 






















Wheat 





.6 


Grain 





.44 


26 


Gifford 


(1979) 


(water limited) 






















Wheat 





.5 


Grain 





.10 


6 


Sionit 


et 


al. 


(1980) 


(1 H^p stress 






















cycle ) 






















Wheat 





.2 


Grain 





.05 


3 


Sionit 


et 


al. 


(1980) 


(2 H20 stress 






















cycles) 






















Wheat 





.1 


Grain 





.02 


1 


Sionit 


et 


al. 


(1981a) 


(1/8 normal 






















nutrient) 























^Calculated from shoots only. 



fertilizer will not prevent any increase in photosynthesis and a 
sequestering of more CC>2 as its concentration increased. 

Proceeding to the yield of grain or cotton lint, one sees increases 
of 0.07 to 0.34% per ppmv increase in C0 2 in optimum growing environ- 
ments. There is no clear evidence that this relative change is less in 
wheat that lacks water f but the wheat deprived of nutrients did respond